Pakistani police carried out a dragnet following a bomb attack on Saturday night that killed a Turkish woman and wounded four FBI agents among several other people dining at a Italian restaurant in Islamabad.

But police said on Monday they have so far been unable to link any those picked up to the attack on the Luna Caprese restaurant, a favourite hangout for foreigners as it is one of the few places in the Pakistani capital where alcohol is served.

The investigation was being carried out jointly with intelligence agencies, and a forensics report was awaited to ascertain whether the bomb was planted in the garden dining area or thrown over a wall.

Investigating officer Ashraf Shah told Reuters that 3-4 kg of explosives were used in the attack.

The blast left a crater about a metre wide, and police were immediately able to rule out a suicide attack, which has become Islamist militants favoured mode of attack since mid-2007.

"We have made some detentions focussing on suspicious people, and people unable to account for their presence in Islamabad... But, there has not been any arrest in the case," Senior Superintendent Kaleem Imam said of the results of the dragnet.

The FBI confirmed on Sunday that four of five Americans wounded in the attack were FBI agents.

"Four FBI personnel were slightly injured in the bombing attack in Pakistan. The FBI is providing the necessary assistance to our employees and their families," FBI spokesman Richard Kolko said.

Kolko provided no further details. The U.S. State Department and embassy in Islamabad had no additional comment.

ABC News identified one of the wounded agents as Ray Biteski, the embassy's FBI attache.

ONE-OFF OR START OF TREND?

It is unknown whether the attack had targeted the restaurant because of the presence of the Americans, or simply because it was a well-known haunt for expatriates, notably diplomats, journalists and aid agency officials.

While Pakistan has reeled from a wave of violence, the al Qaeda inspired militants had until now focused on Pakistani security forces and political leaders, while Westerners and other foreigners have been mostly spared.

It was too early say if the attack was a one-off, or an attack on a specific target, or the start of a trend to pick out soft targets frequented by foreigners.

Foreigners have been attacked several times in Pakistan since the United States persuaded President Pervez Musharraf to join its global war on terrorism in 2001, and U.S. diplomats have been considered most at risk.

The bureau chief of Japan's Kyodo News, Motonobu Endo, was dining in the garden area when the blast occurred, and didn't see anything being thrown.

"I didn't see anything moving before the blast," Endo said.

He recalled seeing a table full of Americans, and reckoned they were sat around three metres from where the bomb exploded, though a couple, including the dead Turkish aid worker, were sat closer to the blast.

While Endo suffered slight injuries and a damaged ear drum, his colleague Toshihisa Onishi suffered eye and facial injuries.

Around a dozen people were wounded, though none suffered life threatening injuries.

A Briton working at the British High Commission was among the wounded, and had since been sent back to Britain, a High Commission spokesman said.

A Canadian and a Chinese national were also hurt, along with three Pakistanis.

The last foreign diplomat to be killed in a militant attack was U.S. diplomat David Foy, who died in a suicide car bomb attack outside the U.S. consulate in Karachi in March 2006. The Karachi consulate had been targeted by bombers several times before.

In 2002, a grenade attack killed five people, including a U.S. diplomat's wife and daughter, at a church in Islamabad's cordoned off enclave for foreign embassies.