Police officers cordon off Toledo Street following an explosion in central Madrid (Bernat Armangue/AP)
At least 11 people were injured in the blast, one seriously, the Madrid emergency service said in a tweet.
The Spanish government’s representative for the Madrid region, Jose Manuel Franco, confirmed three casualties and the Catholic parish that owned the damaged building said the fourth victim was an electrician, a father-of-four, who was working on the boiler and had initially been considered missing.
A police spokesman on the ground told reporters that firefighters were trying to put out a small fire inside the damaged building before they could bring in search dogs and special rescue teams to look for any possible survivors.
A powerful gas explosion tore through a residential building in central Madrid on Wednesday, killing four people and ripping the facade off the structure.