MOSCOW (AP) — Seven Russian servicemen were injured when a World War II-era shell exploded in the basement of a military academy in St. Petersburg on Friday, Russian military officials said.

The condition of the injured in the blast at the Budyonny Military Academy of Communications was not immediately reported. The academy trains soldiers serving in the army’s signal corps.

The military command for the region that includes St. Petersburg said the explosion took place while garbage was being placed in a container during cleaning of a basement in one of the academy’s buildings, Russian news reports said.

The Interfax news agency also cited the command as saying that the explosion was caused by a 76mm shell dating to the 1941-45 period. It was not clear why this ammunition was stored at the academy.