The leader of the private investigative team who has spent years trying to crack the D.B. Cooper hijacking case claimed Thursday he believes the mysterious criminal was a CIA operative whose identity has been covered up by federal agents.

Thomas Colbert, a documentary filmmaker who helped put together the 40-member team, told the Seattle PI his team made the connection from work a code breaker uncovered in each of the five letters allegedly sent by Cooper.

Last month, he insisted in an interview with the PI that a nine-digit number found at the bottom of a letter believed to be sent by the mysterious plane hijacker came from a San Diego man who is still alive -- Robert Rackstraw, an Army veteran.

"The new decryptions include a dare to agents, directives to apparent partners, and a startling claim that is followed by Rackstraw's own initials: If captured, he expects a get-out-of-jail card from a federal spy agency," Colbert said in a news release Thursday.

The FBI released these sketches after a man named D.B. Cooper hijacked a plane flying from Portland to Seattle on Nov. 24, 1971 and then parachuted out the back door with $200,000, never to be seen again (FBI)

The code in question was discovered on the bottom of a fifth letter allegedly sent by Cooper after he hijacked a Northwest Orient Boeing 727 on Nov. 24, 1971.

Colbert said several people who knew Rackstraw have come forward to claim he had possible connections to the CIA and other top-secret operations.

DB COOPER SLEUTH INSISTS LETTER WRITTEN BY MYSTERIOUS HIJACKER LINKED TO SAN DIEGO MAN

The investigator told the newspaper the man who sent the letter may have put the code into the letter to signal to possible co-conspirators that he was alive.

The letter allegedly written by DB Cooper and sent to The Washington Post. (FBI)

Colbert claims that the code refers to three specific Army units Rackstraw was connected to during his military service. One of Rackstraw’s former commanders told Colbert that Rackstraw would have learned some encryption codes before being pulled from the unit.

“I think the coding thing is remarkable, but I'm a hard skeptic,” Dorwin Schreuder, a former FBI agent who worked on the Cooper case in the 1980s, told the Seattle PI back in January. “The circumstances of those codes being what Tom says they are, that he says nobody but him would know these units and these figures, if it's true that's pretty hard to argue against. Rackstraw might be his guy.”

FBI RELEASES 1971 LETTER THAT D.B. COOPER SLEUTH SAYS COULD BE FROM NOTORIOUS HIJACKER

The FBI released the fifth letter in November in response to a Freedom of Information Act lawsuit, which featured the typed number “717171684” opposite the name “Wash Post” in the bottom left corner. Colbert’s team appeared to have linked the number to Rackstraw.

An investigator with Colbert's team claims that in the four other previously released documents, the letters "SWS" appear in one letter, which is short for "Special Warfare School." He also said Cooper claimed to be working for the CIA in another document where the letters "RWR," standing for Robert W. Rackstraw, appear.

Other evidence he complied shows the FBI tried keep the case unsolved.

“As we suspected, records show the Bureau has been stonewalling, covering up evidence and flat-out lying for decades,” he told The Oregonian.

Since last January, the FBI has released more than 3,000 documents to Colbert's team investigating the hijacking. The FBI said in court papers that it has more than 71,000 documents that may be responsive to Colbert’s lawsuit.

Fox News' Robert Gearty and Ryan Gaydos contributed to this report.