Four American tourists were among six people killed when an explosion destroyed a small residential building in central Athens near the Acropolis, officials said Saturday.

The Americans were a family of four who had been staying in an apartment on the building’s first floor. According to Greek media, the family included a couple, their son, and his girlfriend, who were preparing to leave for the airport Friday when the blast occurred. The other victims were an elderly British man and his Greek wife, who lived on the second floor of the three-story building, according to Greece’s state broadcaster ERT.

The explosion occurred around dawn Friday in the Plaka district, one of Athens’ most heavily visited areas. The blast brought down the building and damaged nearby properties and vehicles.

Athens Mayor Haris Doukas said a gas leak was the likely cause, although authorities have not announced a final determination. Investigators continued examining the scene Saturday as emergency crews searched through the debris. Mechanical diggers cleared the wreckage while rescue teams used thermal-imaging equipment. Two neighboring buildings were also considered potentially unsafe.

Doukas described the incident as an “incredible disaster” and said inspections would determine whether the nearby structures could remain standing or would need to be demolished.

Questions about possible gas and electrical hazards in the neighborhood had been raised earlier. A local residents’ association sent a letter to city officials and utility companies about a year ago warning of potential gas leaks and short circuits and calling for a fire drill. A citizens’ group also said a resident on the street had contacted authorities after detecting what appeared to be a gas leak.

Greek authorities had not publicly released the identities, ages, or U.S. hometowns of the four American victims as of Saturday. The blast occurred in a neighborhood popular with international tourists because of its proximity to the Acropolis, which drew approximately 4.6 million visitors last year.