The powerful bombs concealed inside cargo packages and destined for the United States were expertly constructed and unusually sophisticated, said U.S. officials on Saturday, further evidence that Al-Qaeda's affiliate in Yemen is steadily improving its abilities to strike on U.S. soil.

As investigators on three continents conduct forensic analysis of the two bombs and try to piece together a foiled terrorism plot, U.S. officials said evidence was mounting that the top leadership of Al-Qaeda in the Arabian Peninsula, including the radical U.S.-born cleric Anwar al-Awlaki, was behind the attempted attacks.

Investigators said the bomb discovered at the Dubai airport in the United Arab Emirates was concealed in a desktop printer, with high explosives packed into an ink cartridge to avoid detection by scanners.

“The wiring of the device indicates that this was done by professionals,” said one official involved in the investigation. “It was set up so that if you scan it, all the printer components would look right.”

The U.S. officials said their operating assumption was that the two bombs were the work of Ibrahim Hassan al-Asiri, Al-Qaeda in Yemen's top bomb-maker, whose previous devices have been more rudimentary and also unsuccessful. Asiri is believed to have built both the bomb sewn into the underwear of a young Nigerian who tried to blow up a trans-Atlantic flight on Dec. 25, and the suicide bomb that nearly killed Saudi Arabia's intelligence chief, Mohammed bin Nayef, months earlier.

Michael McCaul, ranking Republican on the House homeland security intelligence subcommittee, said federal authorities indicated to him on Saturday that the packages were probably intended to blow up the synagogues in Chicago rather than the cargo planes, since they do not carry passengers. Search

It was a call from bin Nayef, the Saudi intelligence chief, on Thursday evening to John O. Brennan, the White House senior counterterrorism official and former CIA station chief in Riyadh, the Saudi capital, that set off the search. Mr. Bin Nayef also notified CIA officials in Riyadh, said the U.S. officials.

Reviewing the evidence, U.S. intelligence officials say they believe the plot was probably blessed by the highest levels of Al-Qaeda's affiliate in Yemen, including al-Awlaki.

“We know that Awlaki has taken a very specific interest in plotting against the United States, and we've found that he's usually behind any attempt attack on American targets,” said one official.

Obama administration officials said they were discussing a range of responses to the thwarted attack. The failed attack on December 25 created an opportunity for the White House to press Yemen's government to take more aggressive action against Al-Qaeda operatives there, and some U.S. officials believe the conditions are similar now.

A thinly veiled campaign of U.S. missile strikes in Yemen this year has achieved mixed results. U.S. officials said several Al-Qaeda operatives had been killed in the attacks, but there have also been major setbacks, including a strike in May that accidentally killed a deputy governor in a remote province of Yemen.

That strike infuriated Yemen's President Saleh, and forced a months-long halt in the U.S. military campaign.

In recent months, the Obama administration has been debating whether to escalate its secret offensive against the Al-Qaeda affiliate in Yemen. The CIA has a fraction of the staff in Yemen that it has in Pakistan, where the spy agency is running a covert war in the country's tribal areas, but over the course of the year the CIA

has sent more case officers and analysts to Sana'a as part of a task force with the military's Joint Special Operations Command. — New York Times News Service

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