A massive double suicide bombing ripped through a Damascus neighborhood Thursday, killing 55 people, wounding more than 400 and heightening concerns that radical Islamist groups are infiltrating the Syrian uprising.

It was the deadliest bombing yet in the capital and added a sinister dimension to the 14-month-old revolt against President Bashar al-Assad’s rule. The method strongly recalled large-scale attacks by the group al-
Qaeda in Iraq during the height of the violence there.

As details of the Damascus attack emerged throughout the day, U.S. officials said they were increasingly convinced of an al-
Qaeda connection, noting that the twin bombings bore many of the terrorist group’s hallmarks.

According to Syrian state media, two suicide bombers driving cars detonated more than 2,000 pounds of explosives, moments apart, near an intersection on a busy highway during the morning rush hour, a tactic often used in Iraq to penetrate the defenses of key buildings.

In this instance, the target was a headquarters of the Syrian security services’ widely feared Palestine Branch, which was badly damaged in the explosion. The building, which was thought to house many political prisoners, was the target of a rare bombing in 2008 and had since been barricaded off from the nearby highway.

Most of the casualties were civilians caught in traffic as they headed to work or school. State television broadcast grisly images of mangled bodies incinerated in cars and body parts strewn amid the smoldering wreckage of vehicles and piles of broken glass.

“Is this the freedom they want?” one man wailed at the cameras as he held a piece of bloodied clothing. Other witnesses blamed Saudi Arabia and Qatar, the most vocal advocates for the rebel Free Syrian Army, whose fighters are battling government forces in many parts of the country.

Condemnation, blame game

Although there have been several bombings in Damascus and the northern city of Aleppo in recent months, Thursday’s attack was the first to occur on a weekday at a time when the streets are crowded with people. The scenes of devastation, broadcast throughout the day on state television, seemed to signal an ugly twist to the violence that has convulsed the country since the initially peaceful uprising began in March last year.

The attack also raised tensions linked to a fraying U.N.-mandated cease-fire that went into effect April 12 but has failed to halt the violence.

Maj. Gen. Robert Mood, the Norwegian head of the fledgling U.N. mission dispatched to monitor the truce, visited the scene of the attack and called for an immediate end to the bloodshed.

“It needs to stop,” he told reporters. “Whoever inside Syria or outside Syria has been supporting this needs to understand that it is only giving more suffering to the Syrian people.”

U.N. Secretary General Ban Ki-moon “strongly condemned” the attack and urged all parties to the violence to immediately comply with the cease-fire plan, his spokesman said in New York.

Syria map (The Washington Post/The Washington Post)

Kofi Annan, the U.N.-Arab League special envoy who brokered the truce, also urged immediate compliance with the plan. “Any action that serves to escalate tensions and raise the level of violence can only be counterproductive to the interests of all parties,” he said.

The State Department called the bombing “reprehensible and unacceptable.”

The Syrian government blamed “terrorists” and sent a letter to the United Nations demanding immediate Security Council action against “countries, groups and news agencies that are practicing and encouraging terrorism,” a reference to those that have offered support to the opposition.

Throughout the evening, the scenes of the carnage broadcast on Syrian state television were interspersed with images of President Obama, Secretary of State Hillary Rodham Clinton, U.S. Ambassador to the United Nations Susan E. Rice and Israeli Prime Minister Benjamin Netanyahu, as well as British, French and Arab rulers. The montage was accompanied by the slogan, “Their Gift to Syria,” making clear whom the government blames.

The Free Syrian Army issued a statement denying responsibility for the attack and accusing the government of carrying it out.

Ominous signs

Even before Thursday’s bloodshed, however, U.S. and other intelligence officials had justified their reluctance to send materiel support to the rebel movement by citing concerns that militant Islamist groups may be gaining a foothold in Syria.

Jordan and other neighbors have resisted efforts by some countries to supply heavy arms, such as antitank and antiaircraft missiles, to rebel forces, two senior intelligence officials said. Regional leaders are beginning to fear that jihadists may gain de facto control over swaths of territory inside Syria, which could be used to launch terrorist strikes elsewhere.

They cite indications that extremists affiliated with al-Qaeda-linked groups in Iraq have been making their way across the porous Iraqi border into Syria. Regional intelligence officials who closely monitor movement along the border spoke of “hundreds” of experienced al-Qaeda fighters filtering into the country and warned that the terrorist presence could transform the nature and the goals of the uprising.

“They are now inside Syria, and they are well-trained,” said one Middle Eastern official who spoke on the condition of anonymity to discuss sensitive intelligence. “We fear they will turn Syria into what we saw in Iraq after 2004 — a big base for terrorists around the world.”

A previously unknown group calling itself the al-Nusra Front has asserted responsibility for most of the earlier bombings in Damascus and Aleppo, Syria’s largest city. The claims have come in videos posted on militant Islamist Web sites that use language and imagery similar to that used by al-Qaeda in Iraq.

At a briefing in Washington, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said he had no intelligence to suggest that al-Qaeda was responsible for Thursday’s bombings.

“We do know that there are insurgent groups trying to make inroads” into the traditional opposition, he said, adding that there was no reason to believe that it was falling under their influence.

However, two Middle Eastern officials with access to classified intelligence said there is evidence that al-Qaeda-linked groups have carried out some of the recent attacks in Damascus, including a pair of car bombings Dec. 23 that targeted Syrian military intelligence buildings and a Jan. 6 attack against buses carrying riot police.

Activists and Free Syrian Army rebels have described efforts by jihadists to gain a foothold in opposition strongholds, but they said the extremists are not welcome.

State Department spokeswoman Victoria Nuland said that she did not know who was responsible for Thursday’s attacks but that they were “not in keeping” with tactics used by the opposition movement.

“They could be the work of spoilers or other nefarious forces to exploit the situation,” she said.

Warrick reported from Washington. Staff writer Greg Miller in Washington contributed to this report.