NEWLY released FBI documents about the case of a man known as DB Cooper, who hijacked a plane and then vanished without a trace, may only deepen the mystery surrounding the unsolved crime.

This week marks the 46th anniversary of Cooper’s hijacking of a US domestic flight and his subsequent disappearance in a case so baffling the FBI closed the book on it last year.

To mark the anniversary, the FBI has released a letter it was sent back in 1971, which armchair investigators believe was sent by the still-missing Cooper.

“I knew from the start that I wouldn’t be caught,” says the undated, typewritten letter from a person claiming to be the man who said he had a bomb and commandeered a Northwest Airlines flight from Portland to Seattle on November 24, 1971, reports Fox News.

After releasing passengers and crew members, the man then ordered the pilots to fly to Mexico, only to parachute out the back door somewhere over Washington’s rugged wooded terrain with $200,000.

“I didn’t rob Northwest Orient because I thought it would be romantic, heroic or any of the other euphemisms that seem to attach themselves to situations of high risk,” he said.

“I’m no modern-day Robin Hood. Unfortunately (I) do have only 14 months to live.”

The carbon-copy letter was turned over to the FBI three weeks after the hijacking by The Washington Post, The New York Times, The Los Angeles Times and The Seattle Times, which were each mailed a copy and published stories about its contents. The letter was in an envelope with a greater Seattle area postmark.

Last month, the FBI released a copy of the letter that was sent to The Washington Post in response to a Freedom of Information Act lawsuit brought by acclaimed DB Cooper sleuth Tom Colbert, a Los Angeles TV and film producer. He believes the letter is real.

“We have no doubt it’s from Cooper and the reason is that he cites he left no fingerprints on the plane,” he said.

“The reason that’s critical is because it’s absolutely true.”

“There were no prints found in the back of plane. They found 11 partial prints that’s all, sides, fingers, tips and palm. But no prints of value were found.”

The FBI wrapped up its DB Cooper investigation last year without identifying the hijacker or ruling out the possibility that he could have been killed in the treacherous jump.

The bureau said it considered 800 people as suspects.

The FBI also never established the authenticity of the letter to the four newspapers, or, for that matter, four other letters that also purported to be from the hijacker. Those letter were sent a few days after the hijacking.

Investigators got their biggest lead in the case in 1980 when a young boy walking along the Columbia River in Washington found a bundle of rotting $20 bills with serial numbers that matched the ransom money serial numbers.

“My life has been one of hate, turmoil, hunger and more hate; this seemed to be the fastest and most profitable way to gain a few fast grains of peace of mind,” the letter said.

“I don’t blame people for hating me for what I’ve done nor do I blame anybody for wanting me to be caught and punished, though this can never happen.”

The person wrote that he wouldn’t get caught because he wasn’t a “boasting” man, left no fingerprints, wore a toupee and “wore putty make-up”.

“They could add or subtract from the composite a hundred times and not come up with an accurate description,” the letter said, adding, “and we both know it.”

WHAT THE LETTER SAID

The letter, sent in 1971 and just released by the FBI, reads:

“Sirs,

I knew from the start that I wouldn’t be caught.

I didn’t rob Northwest Orient because I thought it would be romantic, heroic or any of the other euphemisms that seem to attach themselves to situations of high risk.

I’m no modern day Robin Hood. Unfortunately do have only 14 months to live (sic).

My life has been one of hate, turmoil, hunger and more hate, this seemed to be the fastest and most profitable way to gain a few fast grains of peace of mind.

I don’t blame people for hating me for what I’ve done nor do I blame anybody for wanting me to be caught and punished, though this can never happen.

Here are some (not all) of the things working against the authorities:

I’m not a boasting man

I left no fingerprints

I wore a toupee

I wore putty make-up

They could add or subtract from the composite a hundred times and not come up with an accurate description; and we both know it.

I’ve come and gone on several airline flights already and am not holed up in some obsure (sic) backwoods town. Neither am I a psychopathic (sic) killer. As a matter of fact I’ve never even received a speeding ticket.

Thank you for your attention.

D.B. Cooper”

A version of this article originally appeared on Fox News and was reproduced with permission.