ATHENS, Greece — An Athens prosecutor ordered an investigation Monday into whether police beat four alleged bank robbers, three of whom are suspected of being members of a domestic militant group that has claimed responsibility in the past for a series of bombings.

The prosecutor ordered the investigation following accusations by relatives of those arrested of torture, and after police released digitally altered photos of the suspects covering signs of beating, leading to an outcry.

The four men, aged 20 to 24, were arrested Friday in northern Greece shortly after a double bank robbery in a village near the city of Kozani. Four other people are being sought in that case, in which the two banks were robbed simultaneously.

Police released mug shots seeking further information on the suspects from the public.

All four photos appeared to have been digitally altered, with signs of bruising showing through the alteration on the left side of three suspects' faces — bruising that was clear in photos taken by photographers during their transfer from the Kozani police station.

Authorities have countered that the suspects had been armed and that they were injured during the operation to arrest them. Public Order Minister Nikos Dendias said on private Mega television that he understood the photos had been altered because police wanted to issue photos that might lead to further information from the public and they needed to be recognizable.

He insisted that instances of torture, if proven, would not be tolerated.

Dendias referred to the suspects as "heavily armed terrorists, terrorists who had carried out a robbery."

Police say two, Yannis Michailidis, 34, and Dimitris Politis, 21, are suspected members of a militant group known as Conspiracy Nuclei of Fire and were on trial in absentia.

The group is best known for a series of mail bombings in 2010 targeting embassies and European officials, including one that reached German Chancellor Angela Merkel's office in Berlin. Neither those nor any of the other bombings claimed by the group caused any injuries.

Police allege they have found the fingerprints of a third suspect, 20-year-old Nikos Romanos, in one of the safe houses used by the group.

Romanos was a close friend of Alexis Grigoropoulos, the 15-year-old shot and killed by a policeman in December 2008 after a verbal altercation in central Athens that led to widespread rioting across Greece. Romanos had been present and witnessed the shooting.

Greece has a long history of small militant groups active in the country. Most target symbols of political power or wealth in bombings and rarely cause injuries.

Last month, a bomb exploded in a popular shopping mall in an Athens suburb on a Sunday morning, causing no injuries. Warning calls were made and the mall, where shops were closed but cafes and movie theaters open, was evacuated before the blast. Two previously unknown groups claimed responsibility.

Anti-terrorism police were searching an apartment in a nearby Athens suburb Monday after indications it had been rented by one of the four arrested in Kozani. Police said items recovered from the apartment included the stock of a shotgun, butane canisters, a flare, cellphone accessories, and a USB stick.