Siler City, N.C. — Siler City fire investigators said Tuesday that they still do not know what caused a blaze that killed two boys and that the investigation could be a lengthy process.

In part, they said, the victims' mother, Maria Inocente Ocampo Barbarosa, had at least five different accounts of what happened and that they question if she was home when the fire started.

"With the inconsistencies in Mom's story, we've got to look into it," Siler City Police Chief Gary Tyson said.

Oswaldo Mendoza, 3, and Jose Manuel Mendoza Ocampo, 1, died Monday at Chatham Hospital of smoke inhalation after the fire broke out at their home at Brookwood Apartments.

Emergency crews found the boys in the upstairs hallway at the entrance of their parents' bedroom. Fire investigators said the blaze started in the downstairs living room, which was heavily damaged.

Authorities said they do not believe electrical problems or cooking were sources of the fire, but they have been unable to determine exactly what happened.

In one version of her story, police said, Ocampo told police she tried to put out the fire but the flames became too much and she could not get upstairs to save the children.

"We don't have any indication that a fire was set by anybody," Siler City Police Maj. William Harman said. "What we don't know is: Who was there when the fire was started (or) how long the fire had burned before anybody was contacted."

A neighbor called 911 at 11:16 a.m., and the fire chief was at the scene within two minutes, officials said. By the time the fire truck arrived six minutes after the call, the first two floors of the building were in flames.

But investigators said Tuesday that a witness reported seeing smoke inside the residence as early as 11 a.m.

Chatham County District Attorney Jim Woodall walked through the charred remains of the apartment with investigators on Tuesday. He had been in Siler City for district court and decided to stop by the scene, he said, but he would not comment on why.

Authorities said there were working smoke detectors inside the apartment and that there were no prior problems with the building.