WASHINGTON -- Counterterrorism officials are warning local law enforcement and emergency personnel to be on the lookout for mail that could have dangerous substances hidden inside.

The FBI and Homeland Security Department say packages from a foreign country with no return addresses and excessive postage need to be scrutinized, according to an advisory sent to local officials around the country and obtained Monday by The Associated Press.

Mail bombs believed to have been designed by the top explosives expert working for al-Qaida in the Arabian Peninsula were sent in packages addressed to Jewish synagogues last week. While officials caught two bombs in the United Arab Emirates and the United Kingdom, U.S. officials say there may be more in the system.