SANDUSKY, Ohio -- Questions are being raised about the circumstances of a Cleveland firefighter's disappearance on Lake Erie over the weekend.

Kenneth "Mike" Alderman fell overboard while boating with his fiancee and friends off the Sandusky shoreline.

Cleveland fire Capt. Bill Graham is among dozens of volunteers searching 15 to 20 square miles of Lake Erie, five days after Alderman disappeared.

"We've got our boats on the water. We've got people walking the shore. We're looking for our brother that is missing. We're not going to leave until we find him," Graham said.

Alderman's friends remain confused as they try to figure out what happened to the experienced boater Saturday night.

The big question is did the boat's propeller injure Alderman?

John Johnston, from the Ohio Division of Watercraft, said three people on the boat initially said Alderman jumped in the lake with no life jacket to help his fiancee out of the water.

Alderman struggled and a friend tried to help, but Alderman went under.

Johnston said the story changed two days later when Alderman's fiancee said he may have been hit by the boat.

"No one said that that night. That might have changed the situation as far as the way you do an investigation. That night, until Monday afternoon, that was never brought up," said Johnston.

Johnston said the owner and operator of the boat, who is also Alderman's friend, denied hitting the firefighter, and investigators said there's no evidence of any criminal wrongdoing.

"Hopefully, they're going to get to the bottom of this, but the main goal right now is to find Mike," said Graham.

Despite the exhaustive search, volunteers have not been able to find Alderman. They will return to the lake on Friday to keep looking.