Two women, a man and a dog were found dead Thursday morning inside a white sport utility vehicle that was spotted partially submerged in a Johns Island creek off Maybank Highway, authorities said.

The Charleston County Coroner’s Office identified the victims as Johns Island residents Don Dent, 50; Mary Wesley Simmonds, 35; and Allyson Key, 38. An autopsy showed all died from drowning, Deputy Coroner Brittney Martin said.

A passerby, Ken Bonerigo of Johns Island, alerted authorities to the partially submerged vehicle in Penny Creek about 7:30 a.m. The creek runs into the Stono River near Headquarters Plantation subdivision between James and Johns Island.

Bonerigo said he suspected that the vehicle crashed into the marsh overnight.

“It’s very dark over there at night and there’s no guard rail there,” Bonerigo said.

Bonerigo said he had hoped that whoever crashed into the marsh managed to crawl out of the water and leave the vehicle behind. He said he later learned from authorities that the vehicle’s occupants weren’t that fortunate.

Charleston police are handling the investigation. Charleston County sheriff’s deputies, St. John’s firefighters, paramedics and crisis chaplains also responded to the scene.

Charleston police spokesman Charles Francis said it was not immediately clear how long the vehicle had been in the creek before the incident was reported. Investigators were still working Thursday afternoon to determine what caused the vehicle to crash, he said.

Emergency crews removed the vehicle from the creek shortly before 10 a.m. The vehicle was later towed from the scene around noon.

More than a dozen relatives of a man who they said was missing showed up at the scene and watched from a distance while authorities assessed the wreckage. One relative said rumors led them the site while they searched for their uncle, who failed to return home Wednesday night from an outing to the nearby St. Johns Yacht Harbor marina. A chaplain was later seen comforting the family.