BOSTON -- Investigators are searching Thursday for the person who scrawled a bomb threat in an American Airlines jetliner bathroom at Logan International Airport, delaying passengers for several hours.

They said they are actually closing in on a suspect and have narrowed a list of potential suspects down to just a few.

Flight 1318 was scheduled to leave Boston for Miami at 2:30 p.m. when the note was found written on a cabinet in a lavatory. No passengers were on board yet.

The airline pulled all the luggage off the plane and had it searched by bomb-sniffing dogs. No explosives were found.

The plane had arrived from Dallas about an hour earlier.

The aircraft was given the all-clear at about 4 p.m., but passengers were transferred onto another plane to continue to Florida, according to Massport spokesman Phil Orlandella.

Earlier this month, An American Airlines flight from Miami to Boston was forced back to Florida because of a suspicious message found in the bathroom. That plane had to turn around and land 45 minutes into the flight.

"(It) more likely (involved) a disgruntled passenger ... because this is a second incident within two weeks," said Edith Flynn, a terrorism expert. "(It's) not likely to be international-motivated terrorism."

She said the incident may also have been labor-related.

Passengers waiting to board the plane said they didn't know what was happening until they saw reports on television monitors.

"They loaded the first people that needed assistance. Then they deplaned them. Then they just said, 'You know what, there's been an issue that's happened," said one passenger.

"We were all sitting in the terminal and CNN came on and we were all sitting there looking at the TV and it looked like our gate and our bags sitting out there. That's how we all found out," said another passenger.