U.S. President Barack Obama made a solemn trip to Dover Air Force Base in Delaware Tuesday, to honor the 30 American and eight Afghan troops killed in a helicopter crash in Afghanistan.

Mr. Obama paid his respects as the remains of the fallen soldiers, including 22 members of the elite Navy SEALs, were brought home on two military cargo planes.

The White House said Mr. Obama later met with about 250 family and friends of the dead who had gathered at the base, and expressed his condolences and gratitude for the troops' sacrifice.

Tuesday's arrival ceremony in Delaware was closed to the media. The Pentagon said that because the military has not been able to identify the remains of any of the victims, including those of the Afghans, next-of-kin were not in a position to grant approval for media access to the transfer ceremony.

Remains of the seven Afghan commandos and an Afghan interpreter will be returned to Afghanistan once they are identified.

Insurgents are believed to have shot down the Chinook transport helicopter Saturday, during an anti-Taliban operation in the remote Tangi Valley of Afghanistan's central Wardak province. But NATO says it does not know the exact cause of the crash.

U.S. Central Command, meanwhile, named Army Brigadier General Jeffrey Colt to lead an investigation into the incident. Central Command said Tuesday the investigation will review the circumstances surrounding the crash.

The crash is the worst loss of life suffered by U.S. forces in a single incident during the decade-long war in Afghanistan.

On Monday, Mr. Obama said the loss of the 30 American troops is “a stark reminder” of the risks that U.S. forces take every day.

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.