The man's name has not been released, but passengers described him as American. Passengers said he started screaming about nine hours into the 15-hour flight about being afraid of the FBI and fearing he was going to be killed. He asked that the flight be diverted to Canada.
"He was clearly not stable," said passenger Jacques Roizen, who helped wrestle the man to the floor and sat in the same row as him after he was restrained. Roizen said he and other passengers and a flight attendant subdued the man when he started reaching for his pockets.
United said it followed its procedures for dealing with disruptive passengers and decided to continue the flight as scheduled.
Passenger Peter Jones said the passenger called out what he said was his name, his birthdate and his Social Security number several times and said he worked for the U.S. Embassy in Abu Dhabi.
Jones said the man claimed to have information about Edward Snowden, a former U.S. government contractor who leaked a once-secret National Security Agency surveillance program two weeks ago.
"He was just saying he had information relating to Ed Snowden and he was being taken back to some safe house somewhere, never to be seen again," Jones said.
A man who said he was on the same flight and recorded the incident with his phone played the audio for reporters. On the recording, a man screamed and repeated "I'm dead" 23 times. The passenger who recorded the clip did not wish to give his name.
"Snowden? No, he's right! I know this now, I know this because of what just happened to me. He's totally right," the man in the recording said.
Jones said he never heard the man threaten passengers or the plane. While the plane was still in the air, the FBI said the man had claimed everyone aboard the plane was being poisoned, but the agency later said it was working to clarify what the man said.
"He said something that people are going to poison him, that he was going to die," Jones said.
On the recording, the man claimed to work for the government.
"You work for the CIA! If you work for the National Reconnaissance Office, you will not get a trial by jury. You will not get a trial by jury. They think I've done something wrong, put me in front of a judge, say I'm guilty," the man on the recording said.
Another passenger, Paula Shea, said the man claimed he was married to a Chinese spy.
Roizen said he did his best to try to calm him.
"He was very paranoid," he said.
Associated Press writer David Porter contributed to this report.