Four American tourists died on Friday after a building collapsed due to a gas explosion in Athens, Greece, in a neighborhood near the Acropolis.

“I am deeply saddened to learn of the death of four American citizens in yesterday’s explosion in central Athens,” U.S. Ambassador to the Hellenic Republic Kimberly Guilfoyle said in a statement Saturday morning. “My thoughts and prayers are with the families and loved ones of those killed and injured in this horrific incident. Our hearts are with them during this difficult time.” A gas leak was initially suspected of causing the explosion, a claim a U.S. State Department official later confirmed to CBS News.