LAC-MEGANTIC, Quebec — Quebec police are pursuing a painstaking, wide-ranging criminal investigation of the inferno ignited by the derailment of a runaway oil train that killed at least 15 people and left dozens missing in the burned-out ruins of a downtown district.

Quebec police inspector Michel Forget ruled out terrorism as a cause, but said Tuesday that an array of other possibilities remain under investigation, including criminal negligence.

Other officials have raised the possibility that the train was tampered with before the crash early Saturday.

“This is an enormous task ahead of us,” Forget said. “We’re not at the stage of arrests.”

The heart of the town’s central business district is being treated as a crime scene and remained cordoned off by police tape — not only the 30 buildings razed by the fire but also many adjacent blocks.