The American Airlines passenger killed by air marshals at Miami International Airport on Wednesday suffered from bipolar disorder, according to investigators, raising new questions about how much training sky marshals receive in dealing with travelers with mental illnesses.

Rigoberto Alpizar, a 44-year-old Costa Rica native and U.S. citizen, was shot multiple times by air marshals after he allegedly claimed to have a bomb in a backpack strapped to his chest, local and federal officials said. The Federal Air Marshal Service said that Alpizar repeatedly ignored the marshals' orders to get down on the ground and that they took appropriate lethal action because they considered him a threat.

After the shooting, police found no evidence of explosives in Alpizar's backpack or in his checked luggage, which was exploded by police on the tarmac as a precaution. "All indications are that this was a textbook situation," said air marshal spokesman David Adams, whose agency is reviewing the incident. "We will also supply results to our training division to see if there's any training changes that need to be made."

Bipolar disorder, also known as manic-depressive disorder, is a mental illness that causes extreme mood swings alternating between euphoria and depression. Left untreated, it can affect rational thinking and can lead to delusions and suicidal behavior.

The image of Alpizar as a potential terrorist threat didn't seem to square with neighbor and family accounts of the Home Depot Inc. employee who lived with his wife on a small street teeming with children in a quiet Orlando suburb. Several neighbors said Alpizar and his wife, Anne Buechner, were friendly and involved in occasional block parties.

"He's a very gentle man, very nice," said Jennifer Tatro, a neighbor who lives on the same street in Maitland, Fla. "I never had an uncomfortable moment with him."

Yesterday afternoon, a woman who identified herself as Alpizar's sister-in-law, Jeanne Jentsch, gave a statement to reporters gathered in front of the Alpizar-Buechner home, saying Alpizar "was a loving, gentle and caring husband, brother and friend." She said Alpizar was born in Costa Rica and became a "proud American citizen several years ago."

Federal officials have not released the names of the two air marshals involved but said they both were based in the Miami field office and joined the agency in 2002. One of the marshals speaks Spanish and formerly worked for the U.S. Border Patrol while the other had worked for the U.S. Customs Service as an officer.

Several passengers aboard the Boeing 757 in Miami said that Alpizar's wife ran after him when he rushed to the front of the aircraft and was confronted by the air marshals. Two passengers, according to media reports, said they heard Alpizar's wife say that he was "sick" or "bipolar" and that he had not taken his medication. Yesterday, Miami-Dade County Police Department investigators said they had interviewed Buechner and that "she advised that Mr. Alpizar had been diagnosed with bipolar disorder."

Lydia Lewis, president of the Depression and Bipolar Support Alliance, a Chicago nonprofit group, said police departments have been increasingly trained to work with mental heath experts in dealing with suspects who are mentally ill. Still, she said, "In this case, it was a terrible tragedy and I'm not certain all the training in the world . . . would have changed this case."

Experts said that police increasingly incorporate mental health training and, in some cases, experts as part of certain units to learn more effective ways of responding to mentally ill people who are not receiving treatment. The air marshals said their training includes a number of different role-playing scenarios involving violent or disruptive passengers as well as hijackers, but a spokesman said he could not say definitively whether the training included how to work with someone who appears to be mentally ill.

"They get trained in all different situations -- not any particular characteristics," Adams said. "A lot of characteristics blend in together" that involve a passenger who is not in a normal state of mind, such as someone who is intoxicated or someone who is mentally ill and has not taken medication, he said. "But air marshals have to react to a situation. When an individual says they have a bomb in the bag, you don't know what someone's mental state is."

Robert Phillips, an adjunct professor of psychiatry and law at the University of Maryland, said it is too difficult in this case to determine whether the marshals acted correctly because the facts are not clear. "Generally, law enforcement is aware that from time to time [mentally ill] individuals need to be handled differently," Phillips said. "There are also circumstances where that can't occur" because of the situation, he said.

Many organizations, including the Air Line Pilots Association and members of Congress, expressed support for the air marshal program yesterday and said the officers acted appropriately. But one anti-gun organization that fought the arming of airline pilots said the incident raises questions about whether passengers are sacrificing safety in the name of security with more guns on aircraft.

"There needs to be a serious investigation," said Kristen Rand, legislative director at the Violence Policy Center. "It seems this man didn't have a bomb and probably wasn't a threat. People think if you put a gun on an airplane, it's going to solve the problem. In this situation, it's created problems."

Catharine Skipp contributed to this report from Miami, and news researcher Magda Jean-Louis contributed from Washington.

Jeanne Jentsch, left, and Steven Buechner, in-laws of Rigoberto Alpizar.