An Afghan policeman on patrol with U.S. soldiers opened fire on the Americans, killing two of them before fleeing, officials said Saturday, raising questions about discipline in the ranks of the Afghan forces and possible infiltration by insurgents.

Training and operating jointly with Afghan police and soldiers is key to the U.S. strategy of dealing with the spreading Taliban-led insurgency and, ultimately, allowing international forces to leave Afghanistan. But Afghan forces have periodically turned their guns on international soldiers.

The U.S. military said two American troops were killed by "an individual wearing an ANP (Afghan National Police) uniform" in Wardak province on Friday. Shahidullah Shahid, a spokesman for the Wardak provincial governor, said the policeman fired on the Americans while they were patrolling together Friday night, killing two and injuring two.

Halim Fidai, governor of Wardak, said two people who recommended the alleged assailant for his job were in custody for questioning. Fidai also said a joint team of American and Afghan officials was investigating the attack, interviewing both the American soldiers and the Afghans who had been on the patrol to learn what happened and how the gunman escaped.

"However tragic, this event will not hamper the close partnership and combined security efforts" of Afghan police and international forces, said Zemarai Bashary, spokesman for the Afghan Interior Ministry, said as part of the U.S. statement on the deaths.

A third U.S. service member died Friday of wounds from a bomb attack in Wardak, the province neighboring Kabul, the day before.

Over a period of less than a month last year, Afghan policemen twice attacked American soldiers in the east. In October 2008, a policeman hurled a grenade and opened fire on a U.S. foot patrol, killing one soldier. In September 2008, an officer opened fire at a Paktia police station, killing a soldier and wounding three before he was fatally shot.

Most recently, in Kabul, an American service member and an Afghan police officer argued because the American was drinking water in front of police during the Ramadan fast, prompting the police officer to shoot the American. Other American troops responded and seriously wounded the Afghan.

In violence Saturday, a remote-controlled bomb on a motorbike exploded in a busy market in northern Kunduz province, killing three Afghans in a region that has recently seen a spike in attacks. Elsewhere in the north, a Finnish convoy hit a roadside bomb in Balkh province, destroying one of the vehicles and injuring four soldiers, Afghan and Finnish officials said.

In western Afghanistan, a Taliban attack on a NATO supply convoy killed a civilian contractor escorting the trucks, said Raouf Ahmadi, a regional police spokesman.

U.S. and NATO deaths dropped in September over the previous two months — possibly due to the Muslim fasting month of Ramadan or because no major offensives were launched. But since President Barack Obama's decision to send 21,000 more troops to curb the growing Taliban-led insurgency, international and civilian tolls have risen steadily.

U.S. forces mounted major operations in July and August in southern Afghanistan to try to dislodge the Taliban from longtime strongholds and improve security ahead of the Aug. 20 presidential election, the outcome of which remains in doubt because of allegations of massive fraud by supporters of President Hamid Karzai.

One of those operations, in Helmand province, has proven to be a relative bright spot, as American and British forces hold territory in a region long plagued by Taliban violence. Lt. Aiden Katz, a Marine platoon commander in Helmand, said his forces came under Taliban fire on Friday while on foot patrol in the countryside.

After a 30-minute firefight with Taliban militants hiding in trees and behind walled-off fields, the Americans called in air support and the gunmen disappeared after Marine Harrier jets strafed the area.

Locals complain that the Taliban "are using their homes, using them to fight Afghan forces," said Katz, 23, of New York City. "We're maintaining pressure on Taliban areas."