The Associated Press . Investigators board a United Parcel Service jet isolated on a runway at Philadelphia International Airport in Philadelphia on Friday. Law enforcement officials are investigating reports of suspicious packages on cargo planes in Philadelphia and Newark, N.J.
WASHINGTON -- Authorities on three continents thwarted multiple terrorist attacks aimed at the United States from Yemen on Friday, seizing two explosive packages addressed to Chicago-area synagogues and packed aboard cargo jets. The plot triggered worldwide fears that al-Qaida was launching a major new terror campaign.
President Barack Obama called the coordinated attacks a ''credible terrorist threat,'' and U.S. officials said they were increasingly confident that al-Qaida's Yemen branch, the group responsible for the failed Detroit airliner bombing last Christmas, was responsible.
Parts of the plot might remain undetected, Obama's counterterror chief warned. ''The United States is not assuming that the attacks were disrupted and is remaining vigilant,'' John Brennan said at the White House.
One of the packages was found aboard a cargo plane in Dubai, the other in England. Preliminary tests indicated the packages contained the powerful industrial explosive PETN, the same chemical used in the Christmas attack, U.S. officials said. The tests had not been confirmed.
In the U.S., cargo planes were searched up and down the Eastern Seaboard, and an Emirates Airlines passenger jet was escorted down the coast to New York by American fighter jets.
No explosives were found aboard those planes, though the investigation was continuing on at least two.
Obama's sobering assessment, delivered from the White House podium, unfolded four days before national elections in which discussion of terrorism has played almost no role. The president went ahead with weekend campaign appearances.
The terrorist efforts ''underscore the necessity of remaining vigilant against terrorism,'' the president said. While he said both packages that contained explosives originated in Yemen, he did not explicitly assign blame to al-Qaida, which is active in the Arab nation and long has made clear its goal of launching new attacks on the United States.
Authorities in Dubai intercepted one explosive device. The second package was aboard a plane searched in East Midlands, north of London, and officials said it contained a printer toner cartridge with wires and powder. Brennan said the devices were in packages about the size of a breadbox.
While Obama didn't specifically accuse Yemen's al-Qaida branch, Brennan called it the most active al-Qaida franchise and said anyone associated with the group was a subject of concern.
The radical U.S.-born Muslim cleric Anwar al-Awlaki, who now is in hiding in Yemen, is believed to have helped inspire recent attacks including the Fort Hood shooting, the Times Square bombing attempt and the failed Detroit airliner bombing last Christmas Day. Another American hiding in Yemen, Samir Khan, has declared himself a traitor and has helped produce al-Qaida propaganda.