January 13, 2005, 5:34 PM EST

MEXICO CITY -- Smoke billowing from the engine compartment led police to discover a large stash of marijuana hidden aboard a luxury bus, the federal Public Security Department said Thursday.

Police had seen the bus stopped along the Mexico City-Cuernavaca highway early Thursday and noticed that smoke was pouring out of the engine area, the agency said in a news release.

They went to offer help, but found no driver or passengers.

When they turned to putting out the flames, the officers discovered packages of burning marijuana that had been hidden in part of the engine compartment.

Police said they estimated that the bus had been carrying about 660 pounds of marijuana.

The case was turned over to federal investigators.