At least three people died and six were injured on Wednesday when a building in central Madrid belonging to the Catholic Church was blown apart by an explosion, local authorities said.

Initial investigations suggested that the blast in Calle Toledo, a street leading out from the city centre, was caused by a gas leak, mayor Jose Luis Martinez-Almeida said.

“The most likely hypothesis is a gas leak, there was a person working on the boiler,” the mayor told reporters as he announced that a body had been discovered under the rubble, raising the death toll to three. “Firefighters still have to retrieve the corpse,” he said, without providing further details.

Earlier, a Church official said one church volunteer was missing. One of the casualties was an 85-year-old woman, authorities said. Two of the injured were transferred to hospital, one in serious condition with a head injury.