Karachi, Pakistan ? FBI agents fanned out Sunday over the site of a deadly car bombing outside the U.S. Consulate, seeking clues as investigators tried to reconstruct how the attack took place.

About 20 Americans most, if not all, FBI agents flown in to help in the probe extensively videotaped and photographed the scene of carnage where 12 people died and 44 were injured in a massive blast Friday.

The death toll rose to 12 Sunday after an injured constable who was guarding the consulate, Ubaid Ullah, died in the hospital, Dr. Seemi Jamali said.

Police said they were taking seriously a claim of responsibility from a previously unknown group called “al-Qanoon,” or The Law. On Sunday, the group said in a fax to the Pakistani newspaper Umat that if President Pervez Musharraf didn’t resign, there would be more attacks.

Although nothing was known of the group al-Qanoon, officials have long predicted that al-Qaida fugitives from Afghanistan and Pakistani religious extremists would launch attacks in revenge for the U.S. war against terror.

Pakistani groups are angry at Musharraf for abandoning Pakistan’s longtime Taliban allies and backing the war on terror after the Sept. 11 attacks in the United States.

Pakistani authorities said they had interviewed many people, including some who were questioned intensively at their homes. They denied that any suspects or witnesses were in custody.

The explosion blew a gaping hole in the heavily guarded consulate’s perimeter wall, shattered windows a block away and sent debris flying a half-mile. The devastation made it difficult to piece together events leading up to the bombing.

Officials first said they thought a suicide bomber was responsible. But attention has shifted to a driver’s training school car that was carrying an instructor and three female students. Police said the bomb may have been stashed in the vehicle by someone who knew it would pass by the consulate and who detonated the explosives by radio from nearby.

The United States closed its consulates in Karachi, Lahore and Peshawar, as well as the American Center in Islamabad, and were considering whether to reopen them today.