U.S. counterterrorism officials suspect that the packages, which were intercepted in Britain and Dubai, were sent by al-Qaeda in the Arabian Peninsula, which had already demonstrated a flair for sophisticated bombmaking in an attempt to blow up a Detroit-bound airliner on Christmas Day.

Obama did not specify any new security measures that would be taken or say that the nation's terrorist alert level had been raised. But he described the threat as serious and pointed once again to Yemen as the source of a terrorist plot, saying his top terrorism adviser, John O. Brennan, had been in touch with that country's president.

The two packages were intercepted at separate locations, one on a UPS plane at East Midlands Airport near Nottingham, England, and the other at a FedEx facility in Dubai, in the United Arab Emirates. Officials provided no details on the kind of explosives found, but they said they were looking at the possibility that the substance used was PETN, the same explosive used in the Christmas Day plot.

A U.S. official said the search for the devices was triggered by a "tip from a very close ally of the United States." The tip was relayed to U.S. authorities Thursday and contained "very specific information" about devices in packages being shipped to the United States from Yemen, the official said.

Brennan, in a statement, said the United States was "grateful to the Kingdom of Saudi Arabia for their assistance in developing information that helped underscore the imminence of the threat emanating from Yemen."

"We were looking for packages that were of concern," Brennan said. He said he briefed Obama before the first package was found in Britain about 3:30 a.m. Friday local time.

U.S. authorities were scrutinizing other packages coming from Yemen, but only the two packages found in Britain and Dubai had been identified as dangerous. Those devices were "intended to do harm," Brennan said.

The plot is certain to reinforce concerns about al-Qaeda in the Arabian Peninsula, or AQAP, an organization in which an American-born cleric, Anwar al-Aulaqi, plays a leading role.

Twice in the past year and a half, AQAP was able to get a PETN-based bomb past security and in position to strike a major target. The first time, in August 2009, one of the group's operatives smuggled an explosive hidden in a body cavity into a meeting with Saudi Arabia's top counterterrorism official. The bomber was killed; his intended target, Prince Mohammed bin Nayef, survived.

Umar Farouk Abdulmutallab, a Nigerian trained in Yemen, had a similar device hidden in his underwear during his alleged attempt to take down the Detroit-bound airliner in December.

Both devices are said to have been developed by AQAP's bombmaking expert, Ibrahim Hassan al-Asiri.

The plot was the second to be broken up this week and followed the arrest of a Northern Virginia man who was alleged to have been planning to bomb Washington's Metro system. The sequence of events illustrated the increasingly dispersed nature of the threats facing the United States. Recent planned attacks have sprung from within the United States and have also been launched by al-Qaeda and its allies from Pakistan and Yemen.

"AQAP has evolved into an increasingly lethal and agile organization, with a proven track record of mounting operations within Yemen, regionally, and internationally," Christopher Boucek, an associate in the Middle East Program at the Carnegie Endowment for International Peace, wrote in a recent article in CTC Sentinel, a publication of the Combating Terrorism Center at West Point, N.Y. "AQAP has been clear in stating its planned objectives, and it has repeatedly delivered on its threats."

Boucek argued that AQAP is more likely to employ "less sophisticated means" to attack the United States than "al-Qaeda central," based in Pakistan, which has focused on "large-scale 'complex' operations."

The latest plot is likely to intensify discussion within the administration about expanding American clandestine operations in Yemen, which have already involved U.S. Special Operations forces and cruise missile strikes. The administration has dramatically stepped up military aid to the country and is weighing whether to deploy armed CIA drone aircraft there.

Coming just four days before the midterm elections, the latest plot also carries political implications. Obama officials have been careful to calibrate their tone on terrorism, refusing to raise the threat level or use what they viewed as the alarmist rhetoric of the George W. Bush administration. But that lower-key approach worked against them after the attempted attack on Christmas Day, when Obama waited three days to make a public statement and his advisers were accused of failing to recognize the seriousness of the incident.

This time, the White House waited less than 24 hours. Obama himself delivered the news that both packages contained explosives during a rare, hastily arranged appearance in the White House briefing room.

Obama aides also faced a question from the Bush era: whether political considerations were at play in their decisions. White House press secretary Robert Gibbs said the seriousness of the threat - and the discovery of explosives - should "put to rest any speculation that may be out there."

White House officials said Obama was alerted to the threat at 10:35 p.m. Thursday.

British authorities had soon flagged a suspicious printer cartridge in a package on a flight stopping over at a UPS hub.

The fear that other packages with explosives might be heading to the United States prompted a flurry of activity Friday, including the search of planes in Philadelphia and Newark and a UPS truck in New York City. An Emirates airlines flight from the United Arab Emirates, carrying cargo from Yemen, was escorted into John F. Kennedy International Airport in New York by fighter jets Friday afternoon.

The package found in Britain contained protruding wires and a white powder. Brennan said that he didn't know how that device or the one found in Dubai were to be activated and that a forensic investigation was ongoing.

finnp@washpost.com millergreg@washpost.com kornbluta@washpost.com

Staff writer Greg Jaffe contributed to this report.

www.washingtonpost.com/wp-dyn/content/article/2010/10/29/AR2010102904395_pf.html

Oct. 29, 2010