Jefferson City. Mo. ? Hours before a fatal fire at a Missouri group home, a maintenance man trying to fix a furnace shorted the electrical wiring running through the attic where investigators believe the fire started, according to documents released Tuesday.

The investigation report by the state fire marshal’s office did not pinpoint an exact cause of the Nov. 27 fire that killed 10 and injured dozens at the Anderson Guest House. But it listed an electrical short or overload in the attic as a possible cause.

The information about the maintenance man’s work on Nov. 26 was included in the fire marshal’s investigation report released to The Associated Press under a state open-records law request. The report labels the investigation as noncriminal.

Documents show that the maintenance man told a fire marshal’s investigator he had used pliers to stick a wire into an open outlet in the furnace room in order to trip the breaker while he worked on the furnace.

Deputy state fire marshal Bill Zieres told the AP on Tuesday that the short-circuit would cause an unusual amount of heat to move through the wires, which in this case ran through the attic.

Investigators have said previously that they found evidence of improper wiring elsewhere in the attic. Poor wiring would be more susceptible to starting a fire when overloaded by a short-circuit, Zieres said.

“That is among the things that could cause damage, but there’s no way for anybody to prove it at this point because of the extent of the fire damage in that area” of the attic, Zieres said.