Paris (CNN) The catastrophic fire at the Notre Dame Cathedral could have been caused by a burning cigarette or an electrical malfunction, French prosecutors said Wednesday.

A preliminary investigation into the April fire , which devastated large parts of the 850-year-old church, found no signs that the blaze was started deliberately, Paris prosecutor Remy Heitz said in a statement.

Investigators carried out a hundred witness hearings during the preliminary investigation, which concluded on Wednesday, Heitz said.

Prosecutors are now looking into the possibility of negligence and said they were opening a judicial investigation.

"If certain failings -- which may explain the scale of the fire -- have been brought to light, the investigations carried out to this date have not been able to determine the causes of the fire," Heitz said.

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