MIAMI - The airline passenger shot to death by federal marshals who said he made a bomb threat was agitated even before boarding and later appeared to be desperate to get off the plane, some fellow travelers said.

One passenger said he "absolutely never heard the word 'bomb' at all" during the uproar as the Orlando-bound flight prepared to leave Miami on Wednesday.

Federal officials say Rigoberto Alpizar made the threat in the jetway, after running up the plane's aisle from his seat at the back of the jetliner. They opened fire because the 44-year-old Home Depot employee ignored their orders to stop, reached into his backpack and said he had a bomb, according to authorities.

Alpizar's brother, speaking from Costa Rica, said he would never believe the shooting was necessary.

[blah blah blah]"I can't conceive that the marshals wouldn't be able to overpower an unarmed, single man, especially knowing he had already cleared every security check," Carlos Alpizar told The Orlando Sentinel.[/blah blah blah]

Some passengers said they noticed Alpizar while waiting to get on the plane. They said he was singing "Go Down Moses" as his wife tried to calm him. Others said they saw him having lunch and described him as restless and anxious, but not dangerous.

"The wife was telling him, 'Calm down. Let other people get on the plane. It will be all right,'" said Alan Tirpak, a passenger.

Some passengers, including John McAlhany, said they believe Alpizar was no threat to anyone.

McAlhany, a 44-year-old construction worker who was returning home from a fishing trip in Key West, said he was sitting in Seat 21C when he noticed a commotion a few rows back.

"I heard him saying to his wife, 'I've got to get off the plane,'" McAlhany said. "He bumped me, bumped a couple of stewardesses. He just wanted to get off the plane."

Alpizar ran up the aisle into the first-class cabin, where marshals chased him onto the jetway, McAlhany said.

McAlhany said he "absolutely never heard the word 'bomb' at all."

"The first time I heard the word 'bomb' was when I was interviewed by the FBI," McAlhany said. "They kept asking if I heard him say the B-word. And I said, 'What is the B-word?' And they were like, 'Bomb.' I said no. They said, 'Are you sure?' And I am."

Added another passenger, Mary Gardner: "I did not hear him say that he had a bomb."

Officials say there was no bomb and they found no connection to terrorism.

Witnesses said Alpizar's wife, Anne Buechner, had frantically tried to explain he was bipolar, a mental illness also known as manic-depression, and was off his medication.

The National Alliance on Mental Illness called on the Air Marshal Service and other law enforcement agencies to train officers if they don't already in responding to people with severe mental illness.

Others said Alpizar's mental health didn't matter while marshals were trying to talk to him and determine if the threat was real.

Shooting to maim or injure _ rather than kill _ is not an option for federal agents, said John Amat, national operations vice president of the Federal Law Enforcement Officers Association, which includes air marshals in its membership.

"The person was screaming, saying he would blow up the plane, reaching into his bag _ they had to react," Amat said.

"The bottom line is, we're trained to shoot to stop the threat," said Amat, who is also a deputy with the U.S. Marshals Service in Miami. "Hollywood has this perception that we are such marksmen we can shoot an arm or leg with accuracy. We can't. These guys were in a very tense situation. In their minds they had to believe this person was an imminent threat to themselves or the people on the plane."

White House spokesman Scott McClellan said the two air marshals appeared to have acted properly when they shot to kill.

Both air marshals were hired in 2002 from other federal law enforcement agencies and were placed on administrative leave, said Brian Doyle, spokesman for the U.S. Department of Homeland Security.

Miami-Dade Police were investigating and the medical examiner's office was performing an autopsy on Alpizar, who was from Costa Rica but became a U.S. citizen years ago. He lived in Maitland, an Orlando suburb.

Neighbors said the couple had been returning to their home from a missionary trip to Ecuador. Buechner works for the Council on Quality and Leadership based in Towson, Md., a nonprofit organization focused on improving life for people with disabilities and mental illness, the organization said in a statement.

David Stempler, president of the Air Travelers Association, said he thinks the shooting may prove more "reassuring than disturbing" to the traveling public his organization represents. "This is a reminder they are there and are protecting the passengers and that it is a seriously deadly business," he said.

Armed police boarded the aircraft after the shooting, with some passengers in hysterics. McAlhany said he remembers having a shotgun pressed into his head by one officer, and hearing cries and screams from many passengers aboard the aircraft after the shooting in the jetway.

"This was wrong," McAlhany said. "This man should be with his family for Christmas. Now he's dead."