KABUL, Afghanistan - Afghan authorities Friday were investigating an overnight raid by NATO troops on a private security company that killed two Afghan guards, an operation that NATO said was conducted after a threat against the U.S. Embassy.

Interior Ministry spokesman Zemeri Bashary said the investigation was looking into why international forces raided the compound of the Afghan company, Tiger International, killing two Afghan guards and wounding two others.

But NATO said the operation was conducted jointly with Afghan forces after receipt of "a credible threat to attack the U.S. Embassy." Coalition forces "coordinated with Afghan security forces" and moved into an area where intelligence reports had located two vehicles thought to be loaded with explosives, NATO said.

The coalition forces were fired on after announcing their arrival, and they returned fire, killing two of the shooters, NATO's statement said.

Fifteen people detained in the operation were released after a senior Afghan army official arrived and "personally vouched" for them, NATO said. A large quantity of weaponry was also seized.

However, Kabul criminal-investigation chief Mohammad Zahir insisted that there had been no coordination with Afghan authorities before the raid, and that the Afghan guards had not opened fire on the coalition troops. The only arms seized, he said, were several weapons belonging to the security guards.

Defense Ministry spokesman Gen. Mohammad Zahir Azimi said his ministry also was unaware of the operation.

President Hamid Karzai has ordered private security companies in Afghanistan to be disbanded - though some will be exempt, such as those protecting diplomatic missions or aid and development projects.

Earlier this week, the Interior Ministry official in charge of the process, Gen. Abdul Manan Farahi, said 57 such firms had already been shut down.

The incident comes after two cases this week in which NATO forces killed Afghan civilians, either during a battle with insurgents or after acting on intelligence relating to suspected militants.

The issue of night raids and civilians killed during operations is particularly troubling for NATO and the Afghan government, and the international coalition says it is careful to avoid such casualties.

In the country's violent eastern province of Khost, a bomb went off Friday on the road between an army base and the nearby town of Khost, killing an Afghan soldier and a passing rickshaw driver, said provincial chief of police Abdul Hakim Esaqzai. He said the blast also wounded another Afghan soldier.

An internal review of President Obama's year-old war strategy, released this month, noted progress against the Taliban in southern Afghanistan - its traditional stronghold, where the United States deployed an additional 30,000 troops this year and where much of the fiercest fighting has been concentrated.

But violence continues across the country, and Khost, which lies on the border with Pakistan, has often been the site of attacks.

Also Friday, NATO said a suspected Taliban "weapons facilitator" detained Dec. 18 in southern Afghanistan was not a member of Iran's powerful Revolutionary Guard.

The statement came after several news reports said the man belonged to a Revolutionary Guard unit responsible for operations outside Iran. Further investigation showed he was not linked to the unit. Iran has been accused of aiding Taliban and other insurgents in Afghanistan.