No bomb was found.
The man, identified as Rigoberto Alpizar, a 44-year-old US citizen, was gunned down on a jetway on Wednesday just before the American Airlines plane was about to leave for Orlando.
"There were no explosives," said James Bauer, who leads the US air marshals field office in Miami.
"There's no reason to believe right now that there is any nexus to terrorism of any other events associated with this one," Bauer said at a news conference a few hours after the shooting.
According to a witness, the man ran down the aisle of the Boeing 757, while his wife tried to explain that he was mentally ill and had not taken his medication.
Plainclothes air marshals on the flight identified themselves and confronted him, but he did not follow their instructions, said Bauer. "As he was attempting to evade them, his actions caused them to fire shots, and in fact he is deceased."
Other officials said the man was shot as he headed over the jetway linking the plane to the airport terminal.
"There's no reason to believe right now that there is any nexus to terrorism of any other events associated with this one"
head of US air marshals office in Miami
The plane, Flight 924, had arrived in Miami from Medellin, Colombia, just after noon (1700 GMT), and the shooting occurred shortly after 2pm (1900 GMT) as the plane was about to take off for Orlando with the man and 119 other passengers and crew, American spokesman Tim Wagner said.
Alpizar had arrived in Miami earlier in the day from Ecuador, authorities said.
Bags blown up
After the shooting, investigators spread passengers' bags on the tarmac and let dogs sniff them for explosives, and bomb squad members blew up at least two bags.
Bauer said no bomb was found and there was no reason to believe there was connection to terrorists.
The concourse where the shooting took place was shut down for half an hour, but the rest of the airport continued operating, officials said.
Mary Gardner, a passenger aboard the Orlando-bound flight, told WTVJ-TV in Miami that the man ran down the aisle from the rear of the plane.
"He was frantic, his arms flailing in the air," she said. She said a woman followed, shouting, "My husband! My husband!"
Gardner said she heard the woman say her husband was bipolar - a mental illness also known as manic-depression - and had not had his medication.
Gardner said four or five shots were fired. She did not see the shooting.
The bomb squad blew up two
bags but found no explosives
After the shooting, police boarded the plane and told the passengers to put their hands on their heads, Gardner said.
"It was quite scary," she told the TV station via a cell phone. "They wouldn't let you move. They wouldn't let you get anything out of your bag."
The Bush administration had hired thousands of air marshals after September 2001, but the exact number is said to be classified.