Before Leon Carroll, Jr. offered his expertise as technical advisor for CBS’ NCIS television franchise, he spent 21 years as a real Special Agent for the Naval Criminal Investigative Service.

Leon began honing the skills he would use as technical advisor for the show when he joined the United States Marine Corps in September 1970. He said he and his friends and classmates began to consider joining when they were still in high school, but what solidified the notion for 16-year-old Leon was the day he observed a Marine in his dress green uniform at church.

“I remember that distinctly because he wore a fourragère,” said Leon. “I thought it was pretty cool because Marines don’t wear a lot of excess stuff on their uniforms. I later learned the French fourragère was awarded to the 5th and 6th Marine regiments in 1918 for their combat at Belleau Wood.”

When he was 17, he approached his mother with a request: sign him up to voluntarily join the Marines. She declined and instead encouraged him to focus on his education. It was one of the best pieces of advice he said he ever received because it led him to college at North Dakota State University.

Leon grew up thinking he wanted to be an architect. However, once he started studying, he realized that he didn’t have the aptitude for all the math and science involved, so he switched and earned a degree in business and economics.

With his education complete, he joined the Marines and went through the Platoon Leader Class Officer Candidate Course and commissioned as a second lieutenant before entering active-duty in 1973. He started as a communications officer with the 3rd Battalion, 7th Marines at Camp Pendleton, California. Then, from 1976 to 1978, he served aboard the USS Ogden (LPD-5) as a combat cargo officer.

Agent for Change

Leon said his interest in law enforcement began to take shape when his mother worked as a clerk at the Chicago Police Department. However, it truly took root after he witnessed police officers who were responding to a shots-fired call in his neighborhood beating a group of young kids who were simply playing basketball.

“These kids had nothing to do with it, but they were thrown into a ‘paddy wagon’ and taken away,” said Leon. “That moment stayed in the back of my head and later pushed me into being an agent for change.”

While still on active duty in the Marines, the idea continued to grow, so he began applying to law enforcement agencies, including the Naval Investigative Service (now NCIS). Some of the agencies accepted his application, but required academy training before his release from active duty, so he declined. The other agencies he applied to did not provide job offers.

Thinking he didn’t get hired because he didn’t have enough education, he returned to NDSU, completed a master’s degree in sociology with a focus on criminology, and tried again.

He Finally Gets his Chance

Leon got a second chance to screen for a NIS position in 1980, this time at the Great Lakes Field Office.

While he waited a man walked by, turned back, and asked, “Don’t I know you?”

Leon recognized him as a senior agent from his first attempt in San Diego. He said the man seemed almost shocked that he had not been hired that first time. The man left and came back a short time later saying someone would be out to retrieve him right away. He said he expected a grueling interview, but after answering only two or three questions, he signed some paperwork and left.

Two weeks later, despite thinking he had bombed the interview, he was offered the job. He wanted to stay in the Great Lakes area because Chicago was his home, but instead he ended up back on the west coast, at his second home in Long Beach, California.

Unlike it is today, training at the Federal Law Enforcement Training Center was not mandatory, so Leon said he was a little surprised to hear that he would be working cases as soon as he arrived in Long Beach.

“My first thought was, ‘Without any training?’ But the thing is, working cases is part of the training. Back in those days, you didn’t immediately go to FLETC. Training was six or seven months of tagging along with another agent, going to crime scenes, and learning whatever that person was doing,” said Leon. “You also spent time researching the manuals.”

To make matters more challenging, agent pay in the 1980s was only about $17,000 a year, which wasn’t enough to cover the cost of living in California. To fill the gap, his boss, Special Agent in Charge Ron Salmon, suggested he join a reserve unit located across the street. Fortunately, the unit needed a communications officer, so for the first three years of his career, Leon balanced the responsibilities of being both an agent and a reservist.

Throughout his career as a Special Agent, he credited several factors for his success, from the marksmanship training he received as a Marine to his experience aboard the USS Ogden, where he learned about Navy culture, to his education in criminology and the insights he gained working with convicted state and federal felons at a prerelease halfway house.

“I got to learn the criminal mind,” said Leon of his time working at the halfway house, “and that really did help me as an investigator.”

Among Leon’s memories of his time at NCIS, he said his first autopsy stands out as one of the most vivid. Just three months into the job, he experienced the unsettling contrast of observing an autopsy on Christmas Day and later watching a turkey being carved during a holiday dinner.

He worked in the same office as Jonathan Pollard, the first NCIS employee sent to prison for sharing top-secret information with Israel. He also served as a Special Agent Afloat aboard the USS Ranger (CV-61) in Thailand and spent two years in Panama after the United States invaded the country.

While in Thailand, he remembers a tragic case where a Sailor was shot and killed by Thai police. The Sailor was on his first day of free time and went out to enjoy himself, meeting a local girl. But the girl’s real boyfriend didn’t like her being with the Sailor. This led to a chase, and the Sailor ran into a police booth for help. Sadly, he couldn’t communicate because of the language barrier, and the situation ended in tragedy.

Leon retired from NCIS on March 3, 2001, coincidentally the same date he had been commissioned in the Marines. He initially planned to spend a few years teaching. Then Sept. 11 happened, so he joined 50 or 60 other retired agents who volunteered to return as rehired annuitants. After the attacks, NCIS was authorized to hire more personnel, including agents, analysts, and other staff, to meet the growing demand for resources. As an annuitant, Leon’s job was conducting background checks and recruiting in an area covering California down to Los Angeles, Nevada, Utah, and Colorado, allowing active agents to dedicate their efforts to counterterrorism and other critical cases.

Opportunity Knocks

While Leon was working as an annuitant at NCIS, Don Bellisario and Don McGill were introducing the characters of a new series they hoped to produce in a two-episode finale of their popular show “JAG.” He said none of his coworkers were aware of the episodes or that their boss, Rick Warmack, was assisting with the production.

The stars aligned for Bellisario and McGill, and “NCIS” was picked up for production by CBS in 2003. In order to make the show seem as authentic as possible, they needed someone familiar with the technical aspects of what an NCIS Special Agent does. For the two “JAG” episodes that was Warmack, but when “NCIS” went into production, they needed someone who could be dedicated to the show fulltime.

“Rick came to me and said, ‘Hey, they’ve come to us and they want someone that can work on a pretty much daily basis. The agency won’t assign anyone active to do that. Since you’re getting ready to re-retire would you be interested in doing it?’” Leon said.

He remembers that he initially balked at the idea because the show was filming in San Pedro, 60 miles from where he lived. But he realized he was already commuting that far every day to get to the NCIS office. The difference was if he worked for the show, he didn’t need to be there every day. That swayed him to accept the offer, and Leon was hired as the technical advisor for the “NCIS” television franchise.

Transitioning from the real NCIS to being the technical advisor for the show wasn’t difficult, Leon said, but it was nothing like he expected.

“I went in thinking they filmed an episode in one day,” he said. “The first day we shot was an 18-hour day for me. At first, I wanted to be there every day even though they may be filming something they didn’t need an advisor for. I did that for the first two weeks and I was exhausted. I thought, ‘What did I get myself into when I signed up for this?’ We would do that for 19 hours a day and then 8 hours later, I had to be in a meeting for the next episode that was going to start filming in seven days.”

The hardest part for Leon was learning what everybody did on the set and how all the pieces intertwined. He figured out pretty quickly though that people were extremely territorial about the part they played in the production… and where they sat. He compared production meetings to meetings in a Navy wardroom. Everyone had “assigned” seating, meaning the seat wasn’t assigned to them specifically, but that was always where they sat.

How Real is Real?

While “NCIS”, now in its 21st season with several spinoffs, is ultimately a television series created for entertainment, Leon’s role as the technical advisor added an element of authenticity. He worked closely with the writers, reviewing scripts and advising them on real-world procedures agents use to help the action feel more believable.

“When I first started, Don Bellisario told people in a meeting, ‘We’re going to do it the way Leon says we do it. As long as it doesn’t hurt the entertainment value, we’re going to follow the procedures the agency actually uses,’” Leon said.

Some of the jargon used in the show was pulled from actual NCIS procedures and manuals, and the pranks characters play on each other in the show are things Leon said happened all the time between agents.

Beyond his role as technical advisor, Leon has made other significant contributions to the show. The character Director Leon Vance, played by Rocky Carroll (no relation to Leon,) shares Leon’s name and his habit of chewing on a toothpick. Additionally, Director Vance’s wife, Jackie, is named after Leon’s own wife. Leon also co-wrote the Season 8 episode ‘Out of the Frying Pan,’ which tells the story of a teenage boy accused of murdering his father.

“I was asked to give 18 pages of interrogation techniques, and the writers used that to put together an outline,” Leon said. “I was involved in pretty much every part of that story, except the casting.”

From the Screen to the Page

Aside from his work with actor Mark Harmon on “NCIS,” Leon has co-authored two books with him. The idea for Leon and Harmon to collaborate on writing was born from their shared desire to highlight real agents on real cases.

Leon said the conversation began around the time season six of the show aired, when the episodes were only loosely connected to Navy and Marine Corps themes. Several years later, Harmon reached out to Leon, and their collaboration took shape.

“He said, ‘Remember what we talked about doing years ago?’” Leon recalls.

“I said, ‘Mark, we’ve sat next to each other for 19 years and had lunch together. We’ve talked about doing a lot of things. What are you talking about?”’ Leon said.
“‘You know — telling real stories. Well, I have an opportunity,’” Harmon explained.

His agent had approached him about writing a book. While the agent likely hoped for a book about the show, Harmon wasn’t interested in that. He told his agent the only book he’d write would be about real agents and their real stories — and he wouldn’t do it without Leon as his cowriter.

They started writing, and “Ghosts of Honolulu” was released in November 2023. The book follows the story of Special Agent Douglas Wada, a Japanese American who worked for NCIS when it was called the Office of Naval Intelligence, as he investigated Takeo Yoshikawa, a Japanese spy gathering intelligence on U.S. Naval operations.

“Unbeknownst to me, two other agents, Matt Parsons and Vic McPherson, had already done extensive research and uncovered a wealth of information. Since Vic was a mentor of mine, I felt comfortable talking to him since I had no clue who Wada was,” said Leon. “I think I connected with Wada though, because, like me, he was a counterintelligence guy.”

Wada’s story was one of five Leon and Harmon proposed, and since WWII stories seem to be popular now, the publisher thought that was the best starting point. It turned out they were right. “Ghosts of Honolulu” has sold more than 100,000 copies and spent three weeks on the New York Times Bestseller list after it was released.

Their second book, “Ghosts of Panama,” released in November 2024, dives into the erosion of the relationship between the U.S. and Panama, and, ultimately, the 1989 Panama invasion. Special Agent Rick Yell was a Naval Investigative Service civilian investigator specializing in criminal cases. Things take a turn when Yell inadvertently develops a relationship with a source who has unfettered access to the regime of Manuel Noriega.

“I was there 13 months after the invasion, and I was reading the case file on it. I saw what our agents had done to prepare for the invasion and thought if I ever get a chance to tell this story, I’m going to do that,” said Leon. “About 25 years later, Mark came to me about partnering to write books about real agents.”

Through everything Leon has done in his life, he said the piece of advice his mother gave him way back when was 17 instilled in him the drive to not quit. He believes that if he had not completed officer training, there is a very good chance that he would not have become a Special Agent, and then he wouldn’t have had the chance to work on the “NCIS” television franchise.

“I’ve been fortunate to have had the opportunity to do all of this. I thank God for my career and what’s happened,” he said. “I’ll be 75-years-old in a couple of months, and my adult life has been awesome!”