Japanese police are questioning staff from the Bulgarian national opera after being notified by the opera company that they are suspected of scrawling graffiti in the Hiroshima Peace Memorial Park.
The park is home to the landmark Atomic Bomb Dome, which was damaged in the 1945 bombing.
Black, spray-painted markings were found on Monday on a bench near the dome, on a fence at the Peace Memorial Museum, and on the door of a garbage disposal facility.
Police say the opera company, which is on a tour in Japan, has told them that some of its staff members may be responsible.
Police also say the graffiti contains what is believed to be the name of a Bulgarian professional soccer club.
The opera company, Sophia Opera and Ballet, told NHK that 2 of its technical staff are to blame for the act of vandalism. It also apologized to the Japanese people for the unconscionable act against the victims of the 1945 bombing.
The Japanese Foreign Ministry says the Bulgarian government offered an apology at the Japanese Embassy in Sofia on Thursday.