Three men are being questioned by Florida police after they were detained in connection with an alert for a potential terrorist threat.

A customer in a Georgia restaurant overheard the trio apparently planning a terrorist attack.

If this was a hoax, my hope is these people would be prosecuted

Jeb Bush Florida Governor

However, federal investigators now believe the comment was a joke aimed at another restaurant patron who had given them a suspicious look, the Miami Herald reported.

The newspaper quoted federal sources as saying the men could be released as early as Friday.

The three men were US citizens - of Iranian, Jordanian and Pakistani descent - and appeared to be medical students going to a conference in Miami.

The alert began after the customer, Eunice Stone, a 44-year-old nurse from Cartersville, Georgia, reported a conversation she overheard, in which the men vowed to make America "cry on 9/13".

Police tracked and stopped the men early on Friday after one of their two vehicles passed through a road toll booth without paying.

Stone overheard "alarming" details

Sniffer dogs and a robot were used to search the vehicles for explosives, and live television showed a bomb squad team blowing up what appeared to be a backpack.

But the search has yet to uncover anything suspicious.

''It appears there isn't a terrorist threat as it relates to destructive devices in the cars,'' Governor Jeb Bush said at a Miami news conference Friday. "If this was a hoax, my hope is these people would be prosecuted.''

White House spokesman Ari Fleischer said it was not clear there was any connection to anything else going on.

But Law Enforcement Department spokesman E.J. Picolo said investigators were "taking it very seriously until we can eliminate the possibility of a threat".

Heightened security

The Georgia Bureau of Investigation asked other police forces to watch out for the vehicles after Ms Stone gave a description including license-plate details.

The cars were pulled over after a sheriff's deputy spotted them in southern Florida.

The occupants apparently refused to immediately allow a search and were described as being uncooperative.

Police shut Alligator Alley - the main east-west route across the Florida Everglades - for several hours.

The US has been on heightened alert against terror threats for the first anniversary of the 11 September attacks.

The BBC's Justin Webb in Washington says there have been a number of similar incidents in recent days, but so far they have all turned out to be innocent misunderstandings or mistakes.