JAMES GORDON MEEK reports:

WASHINGTON - A $625,000 gold shipment vanished early Tuesday in a brazen heist at Miami International Airport after it arrived aboard a jet from Ecuador, police said.

As the FBI confirmed it was leading the hunt for the crooks, the pool of suspects narrowed to the few who had authorized access to the bulk cargo area, federal and police officials with knowledge of the case told ABC News on Thursday.

The gold was spread among six boxes unloaded from American Airlines Flight 902, which arrived at 4:42 a.m. Tuesday morning from Guayaquil, Ecuador, according to a Miami-Dade Police Department incident report.

Video surveillance tapes showed the boxes being unloaded from the plane onto a cart, which was then moved to the opposite side of the airliner at 5:15 a.m., where a small vehicle called a "tug" towed it away.

Police reported that despite the video, it was "unknown who drove the property around to the other side of the plane and left the property there," where the tug then drove the cart off-camera.

The cart was found an hour later - empty.

Those allowed into the restricted bulk cargo area in the airport's international arrivals section include cargo handlers and U.S. Customs and Border Protection officers, but not federal Transportation Security Administration officers, a senior U.S. official said.

"It was an inbound international flight so there was no TSA interaction at all," the senior U.S. official said. "CBP and ramp workers would be the only ones that have contact."

Five "subjects" who worked in the bulk cargo area that morning were interviewed by investigators, but their names were blacked out in the Miami-Dade Police Department's incident report at the request of the FBI, a police official said.

The CBP is the Department of Homeland Security agency responsible for checking inbound cargo arriving from overseas.

CBP spokesmen told ABC News earlier today they were not aware of the incident and referred all inquiries to the FBI.