LULING, La. -- A tugboat sank on the Mississippi River early Tuesday and the vessel's captain is missing.

The Coast Guard said the John 1:1 went down about 4.5 miles south of the Interstate 310 bridge in St. Charles Parish. Two crew members were aboard a barge that the tug was preparing to take control of when the accident occurred.

Petty Officer Tara Mitchell said the boat sank with the captain aboard. He has been identified as Chester Cheramie, 40.

An aerial search lasting more than six hours was called off Tuesday afternoon, but the Coast Guard and local harbor officials continue to search the river with boats.

"Right now, it's too early to determie what the contributing factors were," said Coast Guard spokesman NyxoLyno Cangemi. "Right now, we're concentrating on the search and rescue."

The Coast Guard is investigating why the vessel went down.

The river level is high and the current is especially strong because of upriver snow melt and spring rains, but the Coast Guard did not know whether that was a factor in the sinking of the tug.

"I've seen in my days when it took three or four hours just to get a few miles," said Capt. A.J. Gibbs.

Gibbs has 30 years of experience on the Mississippi and he knows you have to respect it -- especially during times of high water. He said at this time, experience matters.

"Making the turn is more difficult," he said. Manuevering the ships alongside the docks is more difficult because a lot of ships need tug assests at all times."