A U.S. Coast Guard helicopter with four people on board has crashed off Alabama's coast in Mobile Bay on Tuesday night. Three of the crew members are reported missing, while one injured crew member was pronounced dead at the hospital, according to a Coast Guard spokesperson.

The Coast Guard had initially said that it had rescued an injured crew member, but later a press release stated that he was unresponsive to treatment and was pronounced dead. The accident occurred during a training mission by the MH-65 Dolphin, just after 8.30 p.m. near Point Clear, Alabama, Reuters reported. The chopper reportedly went down a couple of miles from the shore in the Mobile Bay.

Though search operations are underway for the missing crew, thick fog in the area has hindered operations. Two additional MH-65s from New Orleans and a response boat from Dauphin Island are assisting in the search operations, Reuters reported.

The flight had originated from USCG Aviation Training Center in Mobile Regional Airport. 

The identities of the missing and the dead crew have not been disclosed due to pending notification of relatives, said the Coast Guard. The cause of the accident is under investigation and no details are known yet.