SEATTLE – The deceased man the FBI is investigating in the D.B. Cooper case was a surveyor whose training might have enabled him to scout locations to parachute from the skyjacked Boeing 727.

Lynn Doyle Cooper, who has been linked to the 1971 skyjacking by his niece, worked as an engineering surveyor, according to an Oregon death certificate.

His brother, Dewy Max Cooper, who also has been implicated by the niece in the skyjacking, once worked at Boeing, his former sister-in-law, Grace Hailey, said Thursday. He also is deceased, Hailey said.

Hailey, 67, who lives in Oklahoma, said she did not know precisely when Dewy Cooper worked at Boeing, but that it was roughly during the period the unsolved skyjacking occurred.

Hailey’s daughter, Marla Cooper, in television interviews Wednesday, said she believes her two uncles were involved in the fabled skyjacking on Thanksgiving Eve 1971.

The FBI said earlier this week that it had been investigating a “promising lead” for more than a year and confirmed that Marla Cooper, who lives in Oklahoma City, had provided information. The FBI said it was working with the family to obtain items from which fingerprints might be lifted to compare to partial prints obtained from the Boeing jet.

The skyjacker parachuted from the Northwest Orient Airlines plane with $200,000 in cash paid by the airline as ransom money. It remains the nation’s only unsolved skyjacking.

Marla Cooper, citing memories from when she was 8 years old, told ABC News this week that she recalled her uncles planning something just before Thanksgiving 1971 at her grandmother’s house in Sisters, Ore. The two used walkie-talkies and left supposedly to go turkey hunting, she said.

On Thanksgiving morning, Lynn Doyle Cooper, known to the family as “L.D.,” returned to the home bloody and bruised, claiming he had been involved in a car accident, Marla Cooper said.

She said she overheard Lynn Cooper say, “We did it, our money problems are over, we hijacked an airplane.”

She said that he vanished after the Thanksgiving gathering and that she was told he had died in 1999.

An Oregon death certificate shows that Lynn Doyle Cooper, 67, died April 30, 1999, and was buried at the Pilot Butte Cemetery near Bend, Ore. His occupation is listed as surveyor and engineering survey.

Hailey, who was married to another of L.D. Cooper’s brothers, Donald, in 1971 before they divorced in 1980, said Thursday she also recalled that L.D. Cooper disappeared and returned only once for a brief visit the next year.

She said Dewy Cooper died of lung cancer 12 to 15 years ago.

Hailey said she did not witness the events described by her daughter on Thanksgiving morning because she had left the house to make pies at the restaurant where Marla Cooper’s grandmother worked.

But Hailey said she recalled that the grandmother, Irene Cooper, was apparently contacted by the FBI shortly after the hijacking.

Irene Cooper called Donald Cooper to ask about L.D.’s whereabouts because of the FBI inquiry, Hailey said.

FBI Special Agent Fred Gutt, a bureau spokesman in Seattle, said the FBI does not discuss specific contacts made during investigations.

“I understand after the hijacking, anyone with the name Cooper in the Northwest was canvassed,” Gutt said. “So that wouldn’t be surprising.”

Hailey said that a few years ago she and her daughter were watching a television program on the D.B. Cooper hijacking.

Hailey said she remarked, “I always did think it was your uncle.”

Marla Cooper then began researching the hijacking and “all these memories started coming back to her,” Hailey said.

In the ABC News interview, Marla Cooper said she was working on a book about L.D. Cooper, but that her primary motivation in going to the FBI was to help solve the case.

Hailey said the two uncles were among five brothers who were outdoorsmen.

She said L.D. Cooper had the physical capability to parachute into the rugged forests of Southwest Washington, where investigators figure the hijacker landed.

The hijacker jumped from the rear of the Boeing 727 on Nov. 24, 1971. Authorities estimated he jumped near the small community of Ariel, Cowlitz County.

He identified himself as “Dan Cooper.” A day after the skyjacking, FBI agents checked out a Portland man with the name D.B. Cooper. Although that man was quickly cleared, the name stuck in the media.