NATCHEZ — Authorities believe a tugboat captain found dead Sunday evening aboard his vessel on the Mississippi River died of natural causes.

Crewmates of the man, whose identity has not been made public, reportedly went to check on the captain after he did not return from his quarters after an unusually long period of time, Mississippi Department of Wildlife, Fisheries and Parks Maj. Lane Ball said.

“My understanding is it was one of the captains, he was there on the (deck) and told one of the employees he was going to go down to his room, and it’s my understanding he never came back,” Ball said. “Some employees went down there to check on him, and they found out he had passed away.”

Ball would not release the name of the deceased and could not provide the name of the company that owns the tugboat.

Ball said a deputy with the Wilkinson County Sheriff’s Office assisted by interviewing the crewmembers on board. Though Ball said the matter remains under investigation, the preliminary findings point to a natural cause of death.

“Early indications (are that) no foul play was involved,” Ball said.

After discovering the captain had died, the crew docked the boat in a remote area just within the Mississippi state line, approximately 3 miles north of Angola.

The crew reportedly contacted their dispatcher in Kentucky, which then contacted the MDWFP, Ball said.

A U.S. Coast Guard representative for the Lower Mississippi River Sector said Sunday that an initial report of the fatality came at 5:15 p.m. that day, but Ball said authorities did not reach the vessel until much later.

Ball received a call regarding the incident at approximately 10:30 p.m.

He said MDWFP officers worked the case well into Monday morning, as both the location of the boat and a low river level created a challenge in reaching the vessel.

At approximately 1:15 a.m. Monday, the officers received a helping hand from the Louisiana State Penitentiary at Angola. The prison supplied a ferry — normally used by the penitentiary’s employees — to reach the vessel and retrieve the body.

The officials, including Wilkinson County Coroner Nolan Thompson, reportedly boarded the ferry at approximately 1:30 a.m. Ball said his officers worked the scene well past 2 a.m., not returning home until approximately 3 a.m.

The investigation remains ongoing, though Ball said the department is essentially waiting on Thompson’s findings to wrap up the case.

“Once we get … the exact cause of death, we will probably be able to close the books on it,” Ball said.

Thompson did not answer phone calls or return messages seeking comment.