Three dogs and three cats inside the home also died, John Fravel said.
Berkeley County Medical Examiner David Brining said Saturday that autopsies of the bodies probably will be performed early this week in Charleston, W.Va.
Brining said his office only will release the identities of the victims if a visual identification can be made. If that is not possible, identities will not be confirmed until an autopsy is performed.
The sheriff's department report stated that the Bedington and Baker Heights volunteer fire departments and firefighters from Williamsport responded to the scene.
When the first firefighters arrived at 1:44 a.m., the mobile home was engulfed in flames, Bedington Fire Chief Scott Schiell said.
"I have no reason to believe it was not an accidental fire," Schiell said. A 911 dispatcher informed units en route of the possibility of people being trapped inside, he said.
The fire was brought under control within a short time, Schiell said. Once it was confirmed that there were bodies inside, Schiell said the sheriff's department and West Virginia State Fire Marshal's Office were notified.
Schiell said the state fire marshal's office is heading the investigation into the cause of the fire. Calls and pages to assistant fire marshals in the county were not returned Saturday.
"I don't believe in the history of the Bedington Fire Department we've ever had five fatalities in one fire," Schiell said.
Only charred ruins were left of the mobile home, in which John Fravel said Clark's family had lived for about a year and a half. Set to one side inside the perimeter of yellow police tape was at least one blackened kerosene heater.
Schiell said it was 18 degrees outside at the time of the fire. Firefighters were on the scene until about 8:20 a.m., he said.
It has been about two decades since there has been a fire this deadly in Berkeley County, said Capt. Eddie Gochenour of the Martinsburg Fire Department. He recalled a fire in late 1987 in the 600 block of West Burke Street in which a couple and their three grandchildren were killed.
Less than a year ago, six people were killed in separate weekend house fires in the Tri-State area. On Dec. 10, 2005, a Greencastle, Pa., couple and their grandson were killed in a fire, and three teenagers died the next day in a fire in Keedysville.