Ocean Isle Beach, N.C. ? A discarded cigarette or other smoking materials may have caused a beach house fire that killed seven South Carolina college students, authorities said Friday, though a preliminary investigation failed to nail down the blaze’s origin.

Mayor Debbie Smith said the probe conducted by the State Bureau of the Investigation and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives confirmed initial reports that the fire started on the back deck and indicated the cause was accidental.

“There has been absolutely no indication that the fire was intentionally set,” Smith said.

But because of the extensive damage, she said, it was impossible for investigators to pinpoint the exact source. Investigators specifically ruled out a grill and an outdoor fireplace, known as a chimenea, as sources, Smith said.

“They may never know where it started,” Smith said. “There’s nothing in the report that tells us what happened.”

Steve Netherland, the special agent in charge of the State Bureau of Investigation’s regional office in Jacksonville, said survivors told investigators there were people smoking at the home. But Netherland declined to say what the students were smoking or speculate further on the fire’s cause.

One of the survivors told The Associated Press this week that students were smoking cigarettes, but that no illegal drugs were in the house.

Aside from the prospect of smoking materials, Smith said, “there have been no reports of any other sources of ignition within that area.”

A final report on the investigation may not be complete for several weeks.