Less than 48 hours after a fire consumed two units of the Parkview Apartment building Monday night, fire officials say they are getting closer to figuring out what happened.

Stephenville Fire Marshal Gary Nabors said two state fire marshals were on hand Wednesday to assist in the investigation. What they concluded, Nabors said, is that the fire started on the outside of the back of the building. Exactly what sparked the blaze, however, is still a mystery.

Nabors said investigators are not ruling anything out, including arson. He said debris from the fire has been sent for testing to help determine the cause of the blaze. He said results will not be ready for several days. Nabors also said that just because it started on the outside of the building doesn’t necessarily mean that it was arson.

“We are not prepared to say how it started at this point,” Nabors said. “We are continuing with the investigation, but we are not ruling anything out.”

Twenty-two fire and rescue personnel from Stephenville, Erath County and Dublin battled the blaze, which started shortly after 8:30 p.m. Monday. Five firefighters were treated for heat exhaustion, but no one was seriously injured.

The apartment complex is made up of two buildings, which include four two-story units each. The families living in the four units that were affected have all been relocated.