In a nervous United States braced for another terrorist attack, the suspicions of a waiter in Georgia were all it took to mobilize five law enforcement agencies after a trio of men.

The three were detained early Friday on a remote stretch of Florida highway. Their cars, a pair of innocuous-looking sedans with Illinois plates, are being searched by bomb robots after dogs alerted police to the possible presence of explosives.

They did not resist police but were being only minimally co-operative, Florida Department of Law Enforcement spokesman E.J. Picolo said. He said that police had identified the three and believed that they are naturalized U.S. citizens.

The men were released later in the day and no charges were laid.

Mickey Lloyd, deputy commissioner of the Georgia Homeland Security Department, said that the waiter, working the lunch shift at a Calhoun restaurant, had become disturbed at the men's conversation.

"She observed some, what she took to be Middle Eastern, men eating," he said. "They made some statements which she heard that were alarming to her."

He refused to elaborate and would not comment on reports that they had been discussing explosives, mocking coverage of Wednesday's remembrance ceremonies and discussing an attack on Miami. According to Reuters police sources, one man had allegedly said that Americans had mourned on 9/11, and would mourn again on 9/13.

A widespread alert was issued, and police throughout the southeastern United States were put on watch for the vehicles. Mr. Picolo told reporters that late at night one of the cars blew through a toll booth at the west end of I-75, heading for Miami, without paying. He said that an officer at the toll booth chased the car down and stopped it about 13 kilometres later on a remote stretch of the road. The other car pulled in immediately behind the cruiser.

Dogs alerted police to the possible presence of explosive material in the cars, and a robot was called in to unpack them. Live TV showed what appeared to be a backpack being blown up by the bomb squad, but police later said they had merely used a water cannon to open it. It reportedly contained various bits of medical equipment.

More than 12 hours after the vehicles had been seized by police, spokespeople announced that the cars had been thoroughly searched, and that no explosive material had been found.

"The dogs are very good at what they do," Miami-Dade Police Sergeant Pete Andrew told reporters earlier, "and obviously we consider this to be a very serious threat."

More than 30 kilometres of I-75, an arrow-straight highway known as "Alligator Alley" that runs across southern Florida from Naples to Fort Lauderdale, just north of Miami, was closed for the investigation.