Four American tourists were among six people killed after a powerful explosion destroyed a residential building near Athens’ famed Acropolis, according to Greek officials.

The four Americans — two men and two women who were believed to be traveling as two couples — were staying in the two-story building in the historic Plaka neighborhood when the blast occurred Friday.

The district sits at the foot of the Acropolis and is surrounded by some of the Greek capital’s most popular tourist attractions.