St. Petersburg investigators have asked prosecutors for the second time to drop a criminal case against two activists from the radical art group Voina, known for its ironic attacks on law enforcement authorities.

Leonid Nikolayev and Oleg Vorotnikov faced up to seven years in prison on hate crime charges over a 2010 stunt dubbed "A Palace Coup," in which they overturned parked police cars, Gazeta.ru said Tuesday.

But investigators decided to drop the case after experts from Herzen State University maintained that the stunt was not a hate crime because the police do not constitute a social group.

The same experts reached the same conclusion in October, but prosecutors refused to approve the request to drop the case.

Nikolayev and Vorotnikov both spent three months in detention before walking out on bail partly paid by British graffiti artist Banksy.

Voina made headlines in June 2010 when they painted a giant penis on a St. Petersburg drawbridge facing a Federal Security Service office to protest the FSB's unchecked powers.