NATO is probing the cause of a helicopter crash in the central Afghan province of Wardak, where 38 coalition members, including 22 personnel from the elite U.S. Navy SEALS, were killed when their aircraft went down early Saturday.

NATO spokesman Brigadier General Carsten Jacobsen said Monday the crash site was being secured and that nobody was being allowed into the area while the probe is ongoing.

The Chinook transport helicopter went down during an anti-Taliban operation in the remote Tangi Valley. Thirty U.S. troops, seven Afghan commandos and an Afghan interpreter died in the incident — the worst loss of life suffered by U.S. forces in a single incident from the decade-long war.

In a statement Monday, NATO said the helicopter was fired on by an insurgent armed with a rocket-propelled grenade. The copter was transporting U.S. troops to the scene of an ongoing battle between coalition forces and insurgents.

In Washington, President Barack Obama said the loss of the 30 American troops is “a stark reminder” of the risks that U.S. forces take every day.

U.S. Defense Secretary Leon Panetta said Monday that as heavy a loss as this was, it would be even more tragic if it were allowed to derail efforts to defeat al-Qaida and deny it a safe haven in Afghanistan.

Pentagon spokesman Colonel Dave Lapan noted the apparent downing of the helicopter by a Taliban rocket-propelled grenade was a single combat incident and did not represent any watershed or trend in the war against the Taliban.

Earlier Monday, another coalition helicopter made a hard landing in the eastern Afghan province of Paktia. NATO said there were no casualties and that there were no initial reports of enemy activity in the area. An investigation into the cause of the incident is under way.

Violence in Afghanistan is at its worst since the U.S.-led invasion in late 2001, with international troop and Afghan civilian deaths reaching record levels.