Four Ukrainian nationals have been charged with last year's attempted murder of Alexei Petrov, a former employee of Bulgaria’s domestic security service DANS.

The names of the suspected attackers were announced by the Prosecutor’s Office at a joint press briefing with the Ministry of the Interior in Sofia on Wednesday.

The presumed organiser of the attack, 46-year old Roman Logvinenko, has been detained upon entering Bulgaria under an order issued by the Sofia City Court.

The other three suspects, who are believed to have carried out the attack, were named as Artyom Tempinskiy, 30, Maxim Chorniy, 30, and 29-year old Yeven Balkov. According to the prosecuting authorities, they had left Bulgaria after the attack and are being searched for under a European Arrest Warrant.

In October 2015, a car owned by Alexei Petrov was hit by a rocket-propelled grenade near his home in Krastova Vada neighbourhood of Sofia.

Petrov and his driver were not harmed in the attack.

The motives behind the attack have not been officially announced.

According to a version, revealed by the Bulgarian public television network BNT on Tuesday, Logvinenko allegedly sought revenge for a ban to stay in Bulgaria imposed on him at the time wneh Petrov had worked for DANS.

In 2010, Petrov was charged with running an organized crime group. The so-called Octopus case was re-launched in March 2015.