SAINT PETERSBURG: A Blast caused by an unidentified explosive device wounded three on Tuesday at a military academy in Russia’s northwestern city of Saint Petersburg, the RIA Novosti state news agency reported, citing the defence ministry.“An explosion of an unidentified explosive device took place around 1:30 pm (1030 GMT), in Saint Petersburg in staff premises of an administrative building at the Mozhaisky academy,” the ministry said in a statement.It described the device as lacking casing, meaning that it was not a military device such as a mortar shell.Three people received non-life-threatening injuries and were receiving medical treatment, it added. Initial reports said that four cadets were injured in the blast.