Police began questioning on a voluntary basis three friends of a 16-year-old girl arrested Sunday for allegedly dumping the body of a former classmate on a mountain in Kure, Hiroshima Prefecture, police sources said Wednesday.

The arrested girl has told the police that she had killed the ex-classmate by herself, but investigators suspect that others — including the three who appeared for questioning — might have been involved in the slaying.

According to the sources, the girl told investigators that on the day she abandoned her classmate's body in the mountains in late June, she and the victim went to the site by car from the city of Hiroshima. Police, however, have already confirmed there was no record of a taxi taking the two girls to the site.