A college student's checked luggage on a Continental Airlines flight from Argentina contained traces of dynamite, authorities said, in one of six security incidents Friday involving U.S. flights.

Federal authorities were investigating why the student, who got off the Continental plane in Houston before it continued to Newark, N.J., had the explosive residue, FBI spokeswoman Shauna Dunlap said.

Houston Fire Department Assistant Chief Omero Longoria said the man told authorities he works in mining and often handles explosives. Longoria said federal officials were investigating whether the explanation was true.

In other incidents:

—A utility knife was found on a vacant passenger seat of a U.S. Airways flight traveling from Philadelphia to Bradley International Airport in Connecticut, state police said.

—A U.S. Airways jet was diverted to Oklahoma City after a federal air marshal reportedly subdued a passenger who was involved in an incident with a flight attendant, officials said.

—An American Airlines flight from England to Chicago was forced to land in Bangor, Maine, for security reasons

—A Continental Airlines Flight for Bakersfield, Texas, was diverted to EL Paso after the crew discovered a missing panel in the lavatory, said Amy von Walter, a Transportation Security Administration spokeswoman.

—An Aer Lingus flight from New York to Dublin was evacuated Friday morning during a scheduled stopover in western Ireland following a bomb threat that turned out to be unfounded, officials said.

In Houston, the dynamite residue was found during a luggage search in a federal inspection station at Bush Intercontinental Airport shortly after Flight 52 landed at about 6 a.m. Marlene McClinton, spokeswoman for the Houston Airport System, said a bomb-sniffing dog "had a hit" on explosive residue during a further search.

She said Customs and Border Patrol and the FBI shut down the customs area and began questioning the passenger who had the luggage.

Dunlap confirmed that the explosive was dynamite. Dunlap also said the man, one of 173 passengers on the flight, was from Connecticut and attends Lafayette College in Easton, Pa. After landing at Newark Liberty International, the plane was kept from the terminal as a precaution, authorities said.

In Connecticut, authorities said they received an emergency call at about 8:25 a.m. reporting that a passenger on U.S. Airways Flight 554 had found the knife and reported it to a flight attendant. When the plane arrived at Bradley, state troopers boarded the aircraft, seized the knife as evidence and interviewed passengers.

No arrests were made and there were no threats involved, said Master Sgt. J. Paul Vance, state police spokesman.

Authorities do not know yet whether a worker inadvertently left the knife on the plane or a passenger brought it on, Vance said.

The FBI was also involved in the investigation.

In another incident, Federal Aviation Administration spokeswoman Arlene Murray said American Airlines Flight 55 out of Manchester, England, was diverted to Bangor for security reasons.

"The TSA learned of a reported threat to the aircraft while it was en route," TSA spokeswoman Andrea McCauley said, declining to give further details.

McCauley said FBI agents were interviewing passengers and crew. She added that officers with dogs trained to detect explosives were also checking the plane.

The Boeing 767 carrying 167 passengers and 12 crew members was diverted at the request of the Transportation Security Administration, airline spokesman John Hotard said.

Hotard declined to say whether any passengers were removed from the flight, but he said the crew needed to be changed because of restrictions on how many hours they can work.

At Oklahoma City's Will Rogers World Airport, law enforcement officers were questioning the passenger involved in the incident that led to the diversion of the U.S. Airways flight, airline spokesman Morgan Durrant said. The Charlotte, N.C.-bound flight had left Phoenix at 6:15 a.m.

Durrant did not disclose the nature of the problem between the passenger and the flight attendant. Officials at the airport and the FBI did not immediately return telephone calls.

A passenger interviewed by a television station said an air marshal tackled the man, but that it wasn't clear what prompted the incident.

Back in Texas, the crew of Continental Airlines Flight 2258 discovered a missing panel in the lavatory, and passengers were being screened and interviewed upon landing in El Paso, the TSA's von Walter said. It was not immediately clear where the flight originated, von Walter said.