PARIS — Investigators are converging around two theories of what caused last week’s devastating fire at Notre-Dame cathedral: a short-circuit near its spire, possibly caused by electrified bells, or negligence by workers carrying out renovations, a theory fueled by the discovery of cigarette butts.

On Thursday, for the first time, police investigators were allowed to search the cathedral’s vast interior for clues, said a police official who was not authorized to speak to the news media and requested anonymity.

The investigators had been prevented from going into the cathedral before Thursday because of concerns about the stability of the damaged structure, whose stone was weakened in the intense heat, the official said.

More than a week after the catastrophic fire shook France and gripped the world, toppling the cathedral’s spire and leading to an outpouring of support, investigators are still seeking a cause of the blaze.