SAN’A, Yemen – Four suspected al-Qaida gunmen blasted their way into the intelligence headquarters of Yemen’s second largest city Saturday and freed several detainees in the group’s most spectacular operation since a U.S.-backed government crackdown began late last year.

The attack on the heavily protected security complex killed 11 and further bolstered U.S. concerns that Yemen’s weak central government may not be up to tackling an increasingly effective foe seemingly able to strike anywhere inside or outside the country.

“We were hit where we least expected it,” Yemeni Information Minister Hassan al-Lozy told the Al-Arabiya news channel. “This is a serious escalation from these terrorist elements.”

U.S. officials say insurgents, including Americans, are training in militant camps in Yemen’s vast lawless spaces and allying with powerful tribes opposed to the government of President Ali Abdullah Saleh.

Those concerns deepened last December, when al-Qaida in the Arabian Peninsula claimed responsibility for the failed attempt to blow up a Detroit-bound airliner.

In the wake of the Christmas attack, with U.S. aid, training and intelligence, Yemen’s military and air force have struck repeatedly at al-Qaida sites and suspected hideouts, and arrested several suspects.

In a statement, the Yemeni government said Saturday’s attack bore the hallmarks of al-Qaida and resulted in the death of seven members of the security forces, three women and a child in the southern port city of Aden.

The fact that one of the most important security institutions in the country’s second largest city could be attacked reflects the state’s weakness, said analyst Mansour Hael, hinting that the attackers must have had inside help.

“The question to ask is how these attackers were able to infiltrate such a fortified security area. This raises a number of suspicions,” he said.

The headquarters of the powerful intelligence agency is located in an upscale neighborhood of government offices overlooking the sea, flanked by the state television building and a branch of the Transport Ministry.

It was the same facility from which 10 prisoners escaped in 2003, including one involved in the plot to blow up the USS Cole in 2000 that killed 17 U.S. sailors.

An eyewitness said the gunmen in military uniforms approached the building after parking their old sedan and a microbus at the nearby historic Crescent Hotel. The witness, who works in the building but was outside at the time of the attack, said the gunmen fired rocket-propelled grenades and threw hand grenades at the building’s entrance before charging inside in a hail of bullets.

In the course of the half-hour fire fight, said the witness, a number of the guards threw down their weapons and fled and the attackers managed to escape with several detainees, leaving the building on fire.

In recent months, the U.S. Defense Department approved spending $155 million to support Yemen’s counterterror operations, including the purchase of four helicopters.

The money also includes $34.5 million to train and equip the Yemeni special forces and another $38 million for aircraft to allow those forces quicker access to hotspots in the country.