Authorities have thwarted multiple terror attacks aimed at the United States, seizing two explosive packages from cargo jets.

The packages were sent from Yemen and addressed to Chicago-area synagogues. The plot triggered worldwide fears that al-Qaeda was launching a major new terror campaign.

President Barack Obama called the coordinated attacks a “credible terrorist threat,” and US officials said they were increasingly confident that al-Qaeda’s Yemen branch, the group responsible for the failed Detroit airliner bombing last December 25, 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 December attack, US officials said. The tests had not been confirmed.

In the United States, 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, although the investigation was continuing on at least two.

Obama’s sobering assessment, delivered from a 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-Qaeda, which is active in that 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 did not specifically accuse Yemen’s al-Qaeda branch, Brennan called it the most active al-Qaeda franchise and said anyone associated with the group was a subject of concern.

Most of the officials spoke on condition of anonymity, citing the continuing investigation.

Brennan later told reporters that the explosives “were in a form that was designed to try to carry out some type of attack,” but he provided no further details.

“The forensic analysis is under way,” he said, adding, “Clearly from the initial observation, the initial analysis that was done, the materials that were found in the device that was uncovered was intended to do harm.”

Intelligence personnel had been monitoring a suspected plot for days, officials said. The packages in England and Dubai were discovered after Saudi Arabian intelligence picked up information related to Yemen and passed it on to the United States, one official said.

Brennan later issued a statement thanking the Saudis for their help and commending British and United Arab Emirates officials for helping prevent violence.

US intelligence officials warned last month that terrorists hoped to mail chemical and biological materials as part of an attack on America and other Western countries using the mail. The alert came in a September 23 bulletin from the Department of Homeland Security that was obtained by The Associated Press.

In the hours following the discoveries, Yemeni officials and Scotland Yard were investigating and the United States issued a 72-hour ban on all cargo from Yemen.

“As a precaution, (the Homeland Security Department) has taken a number of steps to enhance security,” the Homeland Security Department said in a statement. “Some of these security measures will be visible while others will not.”

US authorities conducted searches of aircraft in Philadelphia, Pennsylvania; Newark, New Jersey; and New York City.

Since the failed December 25 bombing attempt aboard a Detroit, Michigan-bound airliner, Yemen has been a focus for US counterterror officials. Before that attack, the United States regarded al-Qaeda’s branch in Yemen as primarily a threat in the region, not to the United States.

The Yemen branch known as al-Qaeda in the Arabian Peninsula has since become a leading source of terrorist propaganda and recruiting. Authorities believe about 300 al-Qaeda members or cells operate in Yemen.

The Yemeni government has stepped up counterterror operations, with help from the US military and intelligence officials. Mohammed Shayba, general-director of the state airline’s cargo department, said the government is conducting an investigation.

“Those in charge are in constant meetings and they are investigating and taking the issue seriously,” he told The Associated Press.