President Barack Obama stopped short of linking the failed plot to al-Qaida in Yemen, but U.S. officials said privately they were increasingly confident that was the source. Obama's counterterrorism adviser John Brennan called the Yemeni wing the most active al-Qaida franchise.
Militants from the terror network have been building up their presence for several years in this nation of 23 million people, finding refuge with tribes in the remote mountain ranges where the government has little control.
But the Yemeni-based al-Qaida offshoot rose to the fore of U.S. concern in December, when it allegedly masterminded a failed Christmas Day attempt to blow up a passenger jet as it landed in Detroit.
The Obama administration branded the group a global threat and has dramatically stepped up its funding of the Yemeni government to uproot it although there has been little visible progress. Friday's discovery of two explosive packages addressed to Chicago-area synagogues and packed aboard cargo jets from Yemen was certain to heighten those fears.
Obama called the coordinated attacks a "credible terrorist threat." Preliminary tests indicated the packages contained the powerful industrial explosive PETN, the same chemical used in the Christmas attempt as well as shoe bomber Richard Reid's effort to blow up a trans-Atlantic flight in 2001, U.S. officials said.
Obama told reporters in Washington that the packages, which were found in England and Dubai, originated in Yemen. He said the group known officially as al-Qaida in the Arabian Peninsula "continues to plan attacks against our homeland, our citizens, and our friends and allies," although he did not directly blame them in the latest attack.
Obama said he had spoken to Yemen's president, Ali Abdullah Saleh, about the incident and pledged continued cooperation with the country. Authorities believe about 300 al-Qaida members or cells operate in Yemen.
For its part, the Yemeni government expressed astonishment at reports linking it to the packages but said in a statement it was cooperating with the U.S. and international investigations. The statement warned against "rush decisions in a case as sensitive as this one and before investigations reveal the truth."
The lukewarm response underscored the dilemma faced by the fragile Yemeni government, which has found itself caught between U.S. pressure to fight the militants and its dependence on the loyalty of unruly tribes that harbor the extremists.
Yemeni authorities also fear too harsh a fight against al-Qaida will alienate a deeply conservative Muslim population where anti-American sentiment is widespread. As a result, the main Yemeni tactic is often to negotiate with tribes to try to persuade them to hand over fugitive militants.
Around 50 elite U.S. military experts are in the country training Yemeni counterterrorism forces - a number that has doubled over the past year. Washington is funneling some $150 million in military assistance to Yemen this year for helicopters, planes and other equipment, along with a similar amount for humanitarian and development aid. San'a says its troops are fanned out around the country, hunting for militants.
Despite the efforts, al-Qaida gunmen have been bold enough to carry out assaults in the capital, San'a, including a failed ambush on a top British diplomat in her car earlier this month. The government touted as a major success a fierce weeklong siege in September by Yemeni troops against an al-Qaida force in the provincial town of Houta, but most of the militants escaped into nearby, impenetrable mountains.
Al-Qaida in Yemen's top leadership also remains largely intact, despite airstrikes that Yemeni officials have said include either coordination from the United States or direct U.S. involvement. American officials have refused to confirm that U.S. planes carried out the strikes.
The hunt for Anwar al-Awlaki, a U.S.-born radical Islamic cleric who Washington says has become a leader in the group, also may have gone cold. The governor of Shabwa province, where al-Awlaki is believed to be hiding in the mountains, told The Associated Press he hasn't been sighted in two months and cast doubt whether the cleric was still in the province.
The United States sees al-Awlaki as the most notorious English-speaking advocate of terrorism directed at America, with a dangerously strong appeal to Muslims in the West, and Washington has put him on a list of militants to kill or capture. U.S. investigators say e-mails link him to the Army psychiatrist accused of last year's killings at Fort Hood, Texas, and that he helped prepare Nigerian Umar Farouk Abdulmutallab, accused in the Christmas airline bombing attempt.
Gamel reported from Cairo. Associated Press writers Hamza Hendawi and Paul Schemm in Cairo and Lolita C. Baldor in Washington contributed to this report.