An explosion that killed six people and injured two others at a northeast Kansas grain elevator was accidental, the State Fire Marshal's Office said Thursday.

The agency said it found no evidence of criminal activity in the Oct. 29 grain dust explosion at the Bartlett Grain Elevator in Atchison, about 50 miles northwest of Kansas City. Preliminary findings indicate the blast apparently originated in the elevator's main bin area. The investigation is ongoing.

Rose Rozmiarek, the chief of investigations for the Fire Marshal's Office, said in a telephone interview that grain dust was definitely the fuel that was ignited, but investigators are still trying to determine what the spark was. The amount of grain dust accumulation at the time is going to be hard to determine, she said.

"All we can say is that there was enough (grain dust) to allow that to happen, which unfortunately is the hazard of any of these elevators," she said. "The elevator itself was half full of product at the time."

Rozarmiarek said she did not know whether the main bin area where the explosion is believed to have originated was the bin being unloaded into rail cars at the time of the blast.

Investigators have completed an external examination of the facility as well as a limited internal look to the extent that those areas are accessible. Some smoldering fire still exists in the damaged bin areas. The grain storage facility has 14 silo areas. About four of them are still inaccessible, she said.

A final report on the explosion will take several weeks to complete, the agency said. The Occupational Safety and Health Administration is conducting its own investigation, which is expected to take several months.

"We are just called in to assist in doing the determination of the origin and cause and -- as long as nothing criminal is involved -- once we have done that, we are done," Rozarmiarek said.

The state investigation is likely to stay open until investigators can get into all areas of the elevator. If some areas remain inaccessible, the state is likely to list the cause as its preliminary findings indicate.

"Unfortunately, we have worked several grain elevator explosions in Kansas," Rozamiarek said. "Nothing is really out of the ordinary per se in this one."