Investigations have begun after US air marshals on board an American Airlines flight on Wednesday shot and killed a man who started acting suspiciously.
US citizen Rigoberto Alpizar, 44, fled before take-off in Miami, saying he was carrying a bomb, officials said.
It was later confirmed that Alpizar had no explosives in his luggage. Witnesses suggested that the Costa Rican-born man may have suffered from mental illness.
The fatal shooting was the first since more marshals were deployed after 9/11.
James Bauer, head of the US air marshal service in Miami, said that controlled explosions carried out on Alpizar's luggage confirmed he was not carrying a bomb.
There was no indication of any link to terrorism, but federal air marshals were deployed in airports around the US as a precaution, Mr Bauer added.
Alpizar had arrived in Miami, Florida, from Ecuador on a flight that had originated in Colombia. He was boarding a flight to Orlando at about 1410 local time (1910 GMT).
Accounts from inside the plane suggest that Alpizar began behaving erratically before he was challenged by the marshals.
"He didn't look stable," fellow passenger John McAlhany told the Miami Herald.
As he ran down the aisle of the plane, a woman assumed to be his wife shouted for him to stop.
Witnesses interviewed after the shooting described how Alpizar's companion tried to tell fellow passengers or air marshals that he suffered from bipolar disorder, or manic depression.
"I did hear the lady say her husband was bipolar and had not had his medication," said Mary Gardner, another passenger.
"I saw the woman... she was hysterical."
Alpizar had been married about 22 years, relatives said
After storming up the aisle of the plane Alpizar reached the aircraft boarding gate, where he was challenged by undercover marshals travelling among the passengers, Mr Bauer said.
At some point Alpizar reportedly suggested that he was carrying a bomb in his backpack, before moving his hand towards the bag.
"The threat escalated," Mr Bauer added, explaining why the marshals then opened fire, killing Alpizar.
Police boarded the plane and ordered fellow passengers to brace themselves against their seats with their hands on their heads.
Sniffer dogs checked passenger luggage on the airport tarmac.
"They put a gun to the back of my head and said: 'Put your hands on the seat'. That was more scary than anything else," passenger John McAlhany said.
In a statement, American Airlines described the shooting as an "isolated incident", adding that none of the other passengers were affected or were ever in danger.
"'I don't know if they shot an innocent man or not. I don't think he was armed or had a bomb. I think he had a mental illness," Mr McAlhany added.
"I don't think they really had to shoot him, but I hope he didn't holler something stupid.''