A car bombing that killed six people outside the Danish Embassy in Pakistan's capital was carried out by a homicide bomber, a senior investigator said Tuesday.

That conclusion could strengthen suspicion that Al Qaeda or an affiliate carried out Monday's blast in retribution for the publication in Denmark of caricatures of the Prophet Muhammad.

On Tuesday, Pakistani investigators searched through the rubble at the scene of the explosion in a leafy neighborhood of Islamabad where security is supposed to be tight. Danish experts were expected to join them.

"I think we can say with a reasonable degree of confidence that it was a suicide attack," Tariq Pervez, director general of the Federal Investigation Agency, told The Associated Press.

Pervez wouldn't say what pointed in that direction.

He said the bomb contained about 55 pounds of explosives and was similar to one used in a suicide bombing in the eastern city of Lahore in March.

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Authorities have provided no results of the investigation into that attack, and Pervez declined to discuss who might have been responsible for Monday's blast.

But the Danish Security and Intelligence Service, known as PET, said the embassy was the likely target.

"It is PET's assessment that Al Qaeda or an Al Qaeda-related group likely is behind the attack," agency director Jakob Scharf said in a statement late Monday.

He added that "a series of other militant Islamic groups and networks in Pakistan also could have the intention and the capacity to hit Danish targets in Pakistan."

The blast wounded some 35 people, left a deep crater on the road outside the embassy and severely damaged a development group's office nearby. The embassy building remained standing, though its windows shattered.

Senior police officer Ahmed Latif said the attacker apparently used a fake diplomatic license plate to get the car near the embassy.

Muhammad Mustafa, one of the Pakistani investigators searching the scene Tuesday, said they were "just trying to find any clue, any evidence. You know yesterday it was panic here. Usually we miss important things in panic."

Danish Foreign Ministry spokesman Klaus A. Holm said there were three Danish diplomats in Islamabad.

"They are working as best as they can in these difficult circumstances, in this building that doesn't really function, so we have to work from other places, other addresses, too," he said.

Denmark has faced threats at its embassies following the reprinting in February by about a dozen newspapers of a cartoon that depicted Muhammad wearing a bomb-shaped turban. That and other drawings of the prophet in a Danish newspaper sparked riots in the Muslim world in 2006.

IntelCenter, a U.S. group that monitors Al Qaeda messages, said Al Qaeda called for attacks against Danish diplomatic facilities and personnel in a video last August, and repeated its threat in April.

The attack could heighten pressure on Pakistan to stop striking peace deals with militants along its border with Afghanistan, where Al Qaeda and Taliban fighters have found sanctuary.

Pakistan insists it is not talking to "terrorists" but rather militants willing to lay down their weapons. But the U.S. has warned that the deals could simply give militants time and space to rebuild strength.

Pakistani officials have implicated Pakistani militants in dozens of suicide blasts including the assassination of former Prime Minister Benazir Bhutto in December. That wave of attacks has eased markedly since the two-month-old government offered peace talks.

Pakistani Prime Minister Yousuf Raza Gilani said in a statement Monday that the blast would "redouble our resolve" to fight terrorism and extremism. However, Interior Ministry chief Rehman Malik said the attack would not affect the peace talks.

Pakistani government officials made no statement Tuesday about the attack.

The six dead included two Pakistani policemen, as well as a cleaner and a handyman employed by the embassy. One of the victims was Pakistani-born with a Danish passport, the Danish government said.

It was the first attack on foreigners in Pakistan since the March bombing of a restaurant in Islamabad that killed a Turkish aid worker and wounded at least 12 others, including at least four FBI personnel.