FBI agents interviewed but did not arrest the passenger aboard the flight from Kona, Hawaii.
"We're not anticipating an arrest," Seattle FBI spokeswoman Ayn Dietrich said late Thursday night.
Two F-15 jets from the Oregon Air National Guard escorted Flight 819 in to Seattle-Tacoma International Airport after a threat call was received at the Honolulu FBI office.
Agents talked to the unidentified man for nearly two hours, Dietrich said. She described him as cooperative.
"I can tell you any threats or tips we receive we take seriously in case they're a credible threat," she said earlier. "We certainly get a number of calls that turn out to be not credible."
The agency's focus now switches to the caller.
A caller told the Honolulu FBI office on Thursday afternoon that a man aboard the flight was a possible hijacker, said FBI spokesman Tom Simon in Honolulu.
Simon declined to provide any details about the caller.
"We reserve the right to investigate what appears to be a hoax phone call," he said late Thursday night. "We'll let the world know if any charges are filed.
"The FBI gets lots of hoax phone calls but something that rises to this level is not something that we're going to take lightly. A hoax phone call of this nature wastes a lot of resources."
Making a false statement to the FBI is a federal crime, he noted.
FBI agents, Port of Seattle police and Transportation Security Administration personnel were waiting for the passenger in Seattle, Dietrich said.
Officers boarded the jet through rear stairs and removed the man from the plane, airline spokesman Paul McElroy said. The passenger had been seated at the back of the aircraft and "slept most of the flight," he added.
The flight crew was aware of the threat but reported no unusual behavior, McElroy said, adding law enforcement made the decision to scramble the military jets.
Passengers deplaned normally after the man was removed. Seattle-Tacoma International Airport spokesman Perry Cooper said airport K-9 dogs then checked the plane but found nothing amiss.