BELLE CHASSE, La. (AP) — Four men remain missing from a commercial cargo ship in the Mississippi River, the U.S. Coast Guard said Wednesday.

The crewmembers were reported missing from the bulk carrier vessel Meghna Adventure around 11 p.m. Monday after they did not arrive for muster, a standard safety drill, the Coast Guard said.

About 494 square miles of the river was searched near Belle Chasse over 12 hours, the agency said. The search was suspended about 6 p.m. Tuesday.

“The decision to suspend a search is one that is taken with great care,” the Coast Guard said in a statement. “The Coast Guard suspended active search efforts after the probable search area was saturated with multiple assets and resources. The search efforts also included multiple agencies conducting shoreline searches as well, however, we were unable to locate the missing individuals.”

It was unclear how the men disappeared.

“The whole situation remains under investigation,” Petty Officer Anthony Randisi said.

The men are described as being from Bangladesh, aged 25, 29, 30, and 47, all with dark hair and brown skin. Their names were not disclosed.