Fighter jets escorted two flights -- one to New York City, another to Detroit -- after passengers' use of the bathrooms aroused suspicions Sunday, the 10th anniversary of the Sept. 11, 2001 attacks.

On a Los Angeles-to-New York American Airlines flight, three passengers made repeated trips to the bathroom, officials said. The three were cleared after the plane landed safely at New York's Kennedy Airport.

Earlier, on a Denver-to-Detroit Frontier Airlines flight, the crew reported that two people were spending "an extraordinarily long time" in a bathroom, Frontier spokesman Peter Kowalchuck said. Police detained three passengers at Detroit's Metropolitan Airport after the plane landed without incident. They were released after questioning.

In both instances, the FBI said the jets shadowed the planes "out of an abundance of caution."

New York, in particular, has been in a heightened state of security after federal officials received a credible but uncorroborated tip of a car bomb plot on the Sept. 11 anniversary in either New York or Washington.

American Airlines spokesman Tim Smith said the plane's captain never declared a security threat and never asked for law enforcement help. A "security concern" was brought to the airline's attention, the crew used "normal procedures" to assess the circumstances and the plane landed as planned, Smith said.

"In our eyes, it's a big nothing," Smith added.

The North American Aerospace Defense Command scrambled two F-16 jets to shadow American Airlines Flight 34 until it landed safely at 4:10 p.m. local time, the Transportation Security Administration said in a statement.

On the flight, the three passengers made repeated trips to the bathroom and some thought they were using hand signals to communicate, a law enforcement official said. The official was not authorized to speak publicly and spoke to The Associated Press on condition of anonymity. Two of the men were Israeli and one was Russian, the official said, adding that they were cleared and sent on their way.

FBI spokesman J. Peter Donald said in a statement that the jets were sent to escort the flight "out of an abundance of caution." The FBI interviewed passengers and found "no nexus to terrorism," he said.

A similar scenario played out on Frontier Flight 623.

NORAD spokesman John Cornelio said NORAD sent two F-16 jets to shadow the plane until it landed safely. The craft, with 116 passengers on board, landed without incident at 3:30 p.m. EDT, Kowalchuck said.

The Airbus 318 taxied to a pad away from the terminal, he said. The three escorted off the plane in handcuffs included two men and a woman, passenger Ilona Hajdar of Michigan told The Associated Press.

Authorities cleared the aircraft at 5:15 p.m. EDT after it was searched, the TSA said.

Flight 623 originated in San Diego before stopping at Denver International Airport on its way to Detroit.

In Denver, the FBI said that NORAD scrambled F-16 fighter jets to shadow the plane "out of an abundance of caution." The plane was searched and nothing was found, the FBI said.

American Airlines passenger Steven Ciobo said nothing seemed amiss on the flight until he saw police lights on the runway after the plane landed. He looked out the window while still in flight and didn't notice anything.

When the plane landed, he said, the airline workers told them to remain seated and that the authorities would meet the plane. Everyone was quiet as air marshals got on board and headed for the back of the plane.

"To be honest, I think it's reassuring that there was such a great response from the authorities," Ciobo said. "If there are people that are stupid enough to do those things on today of all days you wonder what's going on through their heads. But the fact that there were so many authorities there ... and that it all went so smoothly, I think they did a good job.

The jets intercepted the flight about 100 miles west of New York and shadowed it until it landed, Cornelio said. He also described the measure as precautionary.

American Airlines is a subsidiary of AMR Corp. Frontier is a subsidiary of Republic Airways Holdings Inc.