The Moscow prosecutor’s office has ended an investigation into an attack on opposition politician and presidential hopeful Alexei Navalny, citing their inability to find the perpetrators, Navalny wrote on his blog on Thursday.

An attacker threw a green antiseptic, known as zelyonka, at Navalny’s face on April 27 as he was leaving the office of his Anti-Corruption Foundation. The politician partially lost his sight, had to be hospitalized, and later underwent eye surgery in Spain.

The decision to end the criminal case was made on June 13, a day after the Navalny-led opposition held unauthorized anti-corruption protests across the country.

The prosecutors also decided not to pass the criminal case to the Investigative Committee, according to a statement they sent to Navalny’s lawyer, Vadim Kobzev.

Navalny’s supporters later tracked down the attackers and identified them as the activists of the radical pro-Kremlin movement SERB, Alexander Petrunko and Alexei Kulakov. The SERB’s leader Igor Beketov denied the allegations, calling them “provocation.”