ATHENS, Greece (AP) — A Greek prosecutor ordered an investigation yesterday into allegations of police brutality in the arrest of four alleged bank robbers after authorities released digitally altered mug shots that covered up extensive bruising on three of the suspects.

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 two banks were robbed simultaneously. Three of those arrested are suspected of being linked to a militant group that has claimed responsibility for a series of bombings.

Police released mug shots over the weekend 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. The bruising was clear in photos taken during their transfer from the local police station.

Authorities say the suspects had been armed and 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 pictures that could 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.”

There have been numerous allegations of brutality of suspects in Greek police custody in recent years, with complaints of beatings and humiliation.

“The torture of detainees is an embarrassment for the Greek state,” the main opposition left-wing Syriza party said Sunday, adding that such occurrences were “a general phenomenon” in the Greek police and not isolated incidents. “Neither the weight of a criminal act nor any aim of interrogation can justify the torture of detainees.”

Local media quoted the parents of one of the suspects, 23-year-old Andreas Bourtzoukos, as saying their son was forced onto his knees with a hood on his head and beaten for four hours.

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, a 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. They have often used bank robberies as a means of raising funds for their actions.

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 yesterday 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.