An Iraqi prisoner in the custody of U.S. Marines died while being transferred to an American detention facility, military officials said late Thursday.

The prisoner, whose age and other details were not given, died Wednesday while being moved from the city of Rutbah to a military facility called Camp Korean Village. The man had been among 23 people “suspected of insurgent activities” and detained by U.S. forces, a Marine Corps official said.

The arrests were made in a series of “coordinated raids” in the Rutbah area.

“When the convoy arrived at the detention facility, Marines found the man unresponsive. Medical personnel were immediately summoned and rendered aid,” a Marine Corps release read. “However, the man did not respond to treatment and was pronounced dead.”

In previous instances of a detainee’s death, autopsies have been performed before bodies are returned to family members.

“The cause and circumstances surrounding [this] death are under investigation,” the Marine statement read.

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