The researcher, Peter Byrne, then the director of the Bigfoot Information Center and Exhibition in The Dalles, Ore., had a simple question for Jay Cochran Jr., the assistant director of the F.B.I. laboratory division: Have you been testing possible Bigfoot hair samples? And if not, would you like to start?
The 1970s were something of a heyday for Bigfoot researchers — the grainy Patterson-Gimlin film, which claimed to show one of the creatures strolling through a California streambed, was shot in 1967. Mr. Cochran did not seem terribly surprised by the question.
The F.B.I. had been asked several times in the past year whether it had been testing hair samples for possible Bigfoots, Mr. Cochran replied. “However, we have been unable to locate any references to such examinations in our files,” he wrote.
Mr. Byrne had a sample he wanted the F.B.I. to examine. It was 15 strands of hair attached to a small piece of skin that was “the first that we have obtained in six years which we feel may be of importance,” he wrote.
The F.B.I. laboratory was not normally in the business of examining tufts of hair for their potentially fantastical origins — it was more focused on criminal investigations, Mr. Cochran said — but for a reason that may be lost to history, he agreed.
“Occasionally, on a case-by-case basis, in the interest of research and scientific inquiry, we make exceptions to this general policy,” he wrote. “With this understanding, we will examine the hairs and tissue mentioned in your letter.”
Today, the idea of an earnest search for Bigfoot has become the province of reality TV shows like “MonsterQuest” and “Finding Bigfoot.” Not very many people take it seriously. But the 1970s were a different time.
The documents released by the F.B.I. on Wednesday included a long New York Times feature from June 1976 that described Mr. Byrne’s work, including “a handful” of Bigfoot sightings that “hold up and are given high credibility.”
The article, which Mr. Byrne sent to the F.B.I. to illustrate the seriousness of his endeavor, also bemoaned the paltry state of Bigfoot studies in the United States.
The Times said interest in “America’s own ‘monster’” could not hold a candle to the “increasing sums of money” that were “being spent by reputable scientists to investigate Loch Ness.”
In this one instance, at least, it appears that the F.B.I. tried to do its part in the hunt for Bigfoot.
According to the documents released Wednesday, the hairs sent by Mr. Byrne were subjected to a battery of tests, including examinations of root structure, medullary structure and cuticle thickness.
But when the results came back, they were bad news for Bigfoot hunters.
“It was concluded as a result of those examinations that the hairs are of deer family origin,” Mr. Cochran wrote in February 1977. “The hair sample you submitted is being returned as an enclosure to this letter.”
Melissa Hovey-Larsen, the president and founder of the American Bigfoot Society, said she was not surprised that the hair turned out to be from a deer.
“What we hear a lot when we get back hair samples is horse or deer or cow or bear,” she said. “We hear everything. But every so often you get one that comes back and it says ‘unknown source,’ and then nothing ever comes of it from there.”
What was more noteworthy, Ms. Hovey-Larsen said, was that Mr. Byrne turned to the federal government in his search for the truth.
“As researchers go, Peter Byrne blazed more trails to get respect for this field than anyone else in that time period, so I am not shocked he went to the F.B.I. but I am surprised,” she said.
She said most Bigfoot researchers eschew that path.
“As I always say to people, ‘What are they going to tell you?’ First of all, we have no proof that this exists,” she said. “We can’t even get a clear picture. Most of us think we’ll just be laughed right out of the room.”
The documents, which an F.B.I. spokeswoman described as “newly released information,” appeared to be the first time that federal law enforcement had acknowledged conducting any Bigfoot-related inquiry.
The spokeswoman said the release of the documents on Twitter was not intended to be an “X-Files”-style big reveal.
The account that published them, @FBIRecordsVault, automatically tweets documents that have been entered into the agency’s Freedom of Information Act library after a successful FOIA request, she said.
Some at the agency were amused at the public interest sparked by the documents and the cryptic tweet that announced their arrival.
“Oh, my God,” a receptionist at the F.B.I. press office said to a reporter who called to ask about Bigfoot. “I cannot believe that is why you are calling.”