Four suspects arrested last week for a double bank heist in northern Greece have been linked to the urban guerrilla group Conspiracy of the Cells of Fire, according to police who believe the four men could be the perpetrators behind a bloodless bomb blast at The Mall in northern Athens last month.

Police said Monday that the fingerprints of two of the suspects had been found in two apartments – in the Athens neighborhoods of Kallithea and Halandri – believed to have been used by the guerrilla group as hideouts. The fingerprints of Andreas Bourtzoukos, 23, were found in the Halandri flat, and those of Nikos Romanos, 20, in a flat in Kallithea. Romanos, who was a key witness in the trial of the murder of Alexis Grigoropoulos, the teenager who was shot dead by a policeman in December 2008, reportedly used a false identity card, under the name Constantinos Giakoumopoulos, to rent out a second apartment in Halandri, according to police. That apartment was searched by police late on Sunday night, according to sources who said the property bore signs of a break-in, possibly by members of the group attempting to destroy evidence. Officers searching the second Halandri flat reportedly discovered plastic bags identical to the ones in which an explosive device planted in The Mall had been transported last month. (An unknown organization claiming to be the result of a partnership between anarchist groups “Wild Freedom" and “Instigators of Social Explosions” claimed responsibility for The Mall bombing at the end of last month.)

The other two robbery suspects arrested last Friday – Yiannis Michailidis, 25 and Dimitris Politis, 22 – had already been linked to Conspiracy of the Cells of Fire. Warrants for their arrests had been issued in 2011. The pair were transferred Monday to a specially designed courtroom at Attica’s high-security Korydallos Prison where 17 people are on trial for membership of the group.

Meanwhile Athens public prosecutor Panagiota Fakou launched an investigation into claims that all four suspects were beaten while in police custody. The intervention followed complaints by relatives after photographs of the four suspects, bearing heavy bruises to their faces, were published by police. Indications that some of the photographs were doctored, to conceal the presence of police officers’ hands constraining the suspects or to tone down the bruising, provoked strong criticism.

Public Order Minister Nikos Dendias said the injuries were sustained during the suspects’ arrest but promised “merciless punishment” if any officers are found to have abused the suspects in custody. He said the photos had been doctored so the suspects would be recognizable to the public.

Leftist opposition SYRIZA and

Democratic Left, the junior coalition partner, called for a probe into how the detainees sustained their injuries.

Romanos said he would not sue the police, claiming to be a “prisoner of war.” “I want the abuse I suffered to prick citizens’ consciences.”