MIAMI, Florida (CNN) -- A 44-year-old U.S. citizen who claimed to have a bomb was shot and killed when air marshals opened fire on a boarding bridge at the Miami airport, several sources told CNN. No bomb was found.

American Airlines Flight 924 was in Miami on a stopover during a flight from Medellin, Colombia, to Orlando, Florida, when the man, identified as Rigoberto Alpizar, said there was a bomb in his carry-on backpack, a Department of Homeland Security official said.

Alpizar was confronted by a team of federal air marshals, who followed him down the boarding bridge and ordered him to get on the ground, the official said. ( Watch the aftermath of the shooting -- 1:36)

When Alpizar appeared to reach into his backpack, he was shot and wounded, the official said, adding that the marshals' actions were consistent with their training. Officials said later that the man died of his injuries. ( Watch an air marshal talk about his extensive training -- 5:03)

Upon investigation, there was no evidence that Alpizar had a bomb, an official said.

Alpizar was traveling with a woman and had arrived in Miami on a plane from Quito, Ecuador, federal officials said. He and the woman began arguing before getting off the plane in Miami, two officials said.

A passenger, Mary Gardner, told WTVJ in Miami that the man ran frantically down the aisle from the rear of the plane, arms flailing, and that the woman accompanying him said that her husband was bipolar and had not taken his medication, according to The Associated Press.

After he got off the plane in Miami and went through customs, he got on the Orlando-bound plane and said he had a bomb, Air Marshal Service spokesman Dave Adams said. (Read about air marshals being taught to avoid risk)

Air marshals asked him to get off the plane, which he did, but when they asked him to put his bag down, he refused, Adams said. Alpizar then approached the marshals in an aggressive manner, at which point two or three shots were fired, he said.

Karlina Griffith, translating for her grandmother, witness Miriam Delgado, told WFOR television that Delgado heard three gunshots before people started running and "going crazy." ( Watch a witness account -- :40)

Officials could not confirm if Alpizar suffered from mental illness. His mother-in-law told WKMG television in Orlando that he suffered from bipolar disorder, but his brother-in-law, Steven Buechner, told CNN he was unaware of any mental problems.

Alpizar moved to the United States from Costa Rica in 1986 and worked for Home Depot, Buechner said. He and his wife had been in South America since the day after Thanksgiving to help her uncle, a volunteer dentist, Buechner said.

Alpizar and his wife lived in Maitland, Florida, just a few miles north of Orlando, and they had no children, Buechner said.

The killing marks the first time a federal air marshal has fired a weapon at an individual since the program was bolstered after the September 11, 2001, attacks.

Footage from the scene showed armed SWAT team members carrying rifles outside the aircraft, along with more than a dozen police vehicles. Paramedics were standing on the stairway to the aircraft.

Investigators took the backpack and two other pieces of Alpizar's luggage onto the tarmac, and an explosives team blew the bag open by firing a bottle full of water at it. The water is used to effectively defuse any explosive device by separating its components.

The Boeing 757, which can hold about 180 passengers, was due to take off for Orlando at 2:18 p.m. ET. It had arrived in Miami at 12:16 p.m. ET, according to the airline's Web site. No other flights at Miami International were disrupted Wednesday, an airport official said.