(CNN) -- Cargo planes and trucks in several U.S. cities were inspected Friday after investigators found suspicious packages in at least two locations abroad, law enforcement sources with detailed knowledge of the investigation said.
U.S. officials believe that al Qaeda in the Arabian Peninsula, commonly referred to as AQAP, is behind the plot.
One suspicious package, found in the United Kingdom, contained a "manipulated" toner cartridge but tested negative for explosive material, the source said. It led to heightened inspection of arriving cargo flights in Newark, New Jersey, and Philadelphia, Pennsylvania, and a UPS truck in New York.
The package had white powder on it as well as wires and a circuit board, a law-enforcement source said; someone shipped it from Sanaa, Yemen, with a final destination of Chicago, Illinois. A similar package has been discovered in Dubai, in the United Arab Emirates, the source said.
Investigators were looking for a "possible nexus to terrorism," a U.S. official said.