Afghanistan's president ordered an investigation Tuesday into a bombing that killed 21 civilians at a wrestling match — the deadliest homicide attack since U.S.-led forces ousted the Taliban in 2001.

President Hamid Karzai ordered the inquiry to focus on "where the militants are getting their resources, their support and where they are coming from," said Khaleeq Ahmed, one of his spokesmen.

Hours before Monday's explosion in a town on the Afghan-Pakistan border, Karzai told reporters he believed most of those responsible for about 20 homicide attacks in the past three months have been foreigners, though he did not say from where.

Afghan officials repeatedly claim that the Taliban and other militant groups have training bases in Pakistan and are receiving support from there — an accusation Islamabad denies.

On Tuesday, Pakistani provincial authorities said as many as five foreign terrorists were killed in a purported U.S. missile strike last week on the Pakistan side of the border.

Pakistani intelligence officials have said the target of the attack was al-Qaida's No. 2, Ayman al-Zawahri, who they said was invited to a dinner celebrating an Islamic holiday in the village but sent aides instead.

Eighteen residents, including women and children, were also killed in the strike, the provincial government said Tuesday.

In Monday's attack in Afghanistan, an unidentified man blew himself up after driving into a crowd of about 100 wrestling fans watching a bout in Spinboldak, a key crossing point into Pakistan.

Kandahar Gov. Asadullah Khalid said he was certain that the bomber had come from Pakistan.

"We have started searching for his accomplices, but it's no good as they've almost definitely fled already back to Pakistan," he told The Associated Press.

In his comments to reporters, Karzai warned that Afghanistan could again become a staging post for terrorist strikes in Europe and America if international support wavers.

Violence in southern and eastern Afghanistan spiked last year, leaving about 1,600 people dead.