The agency came to its conclusion after speaking with parachute experts and digging where children found the parachute early last month, said Laura Laughlin, special agent in charge of the FBI's Seattle division.
Earlier, the man who packed the four chutes given to the mysterious hijacker said they could not have been used by Cooper. Earl Cossey examined the found parachute for the FBI on Friday.
He told The Columbian of Vancouver that the newly found chute "absolutely, for sure" could not have been one of the four that he provided.
"The D.B. Cooper parachute was made of nylon," he said. "This 1945 parachute was made of silk."
Cossey sold parachutes at a skydiving operation in the 1970s and provided the chutes that the FBI gave Cooper.
Agents found more fabric and parachute lines as they dug at the site, but no harness, which would have provided a serial number and possible source of the find, FBI spokeswoman Robbie Burroughs said.
Thelast fall, hoping to generate new tips to solve the 36-year-old mystery. The torn, tangled parachute was found about a month ago by children along a dirt road near Amboy.
A man who gave his name as Cooper hijacked a Northwest Orient flight from Portland, Ore., to Seattle in November 1971, claiming he had a bomb.
After the plane landed at Seattle, he released the passengers in exchange for $200,000 and four parachutes and asked to be flown to Mexico. He then bailed out of the jet as it flew somewhere near the Oregon line.
Some of the cash has been found but his fate is unknown, and investigators doubt he survived.