(Reuters) - Jan 21 (Reuters) - Six U.S. soldiers who died in a helicopter crash in southern Afghanistan were identified by the U.S. Defense Department on Saturday.

The Marines killed were Captain Daniel B. Bartle, 27, of Ferndale, Washington; Captain Nathan R. McHone, 29, of Crystal Lake, Illinois; Master Sergeant Travis W. Riddick, 40, of Centerville, Iowa; Corporal Jesse W. Stites, 23 of North Beach Maryland, Corporal Kevin J. Reinhard, 25, of Colonia, New Jersey; and Corporal Joseph D. Logan, 22, of Willis, Texas, according to a statement from the Defense Department.

The cause of the crash is under investigation, a spokesman for the NATO-led International Security Assistance Force said Friday.

He said initial reporting indicates there was no enemy activity in the area at the time of the crash, raising questions about a possible malfunction or pilot error.

It is the worst crash since August of last year when 30 American forces, including 22 elite Navy SEAL commandos, died when their helicopter came down in eastern Afghanistan. Eight Afghans aboard were also killed.

An investigation into that incident confirmed that the helicopter was brought down by a Taliban rocket-propelled grenade.

(Writing by Mary Wisniewsk; Additional Reporting by Rob Taylor; Editing by Greg McCune)