CCTV footage from The Mall in Maroussi, where a bomb blast injured two security guards on Sunday, suggests that four people were involved in planting the device.
According to reports, officers have identified four people wearing hoods or baseball caps who are thought to have left the bomb in a newspaper stand.
Police believe that as many as eight people in total may have taken part in the attack.
Remnants of a pressure cooker was found in the wreckage created by the bomb, which is thought to have contained ammonium nitrate and fuel oil (ANFO) and gelignite.
The urban guerrilla group Conspiracy of the Cells of Fire has carried out attacks using pressure cookers to construct explosive devices in the past but authorities are not making any direct connections at this point.
No group has claimed the attack so far.
The bomb was left in a newspaper stand and an anonymous caller left a warning with Eleftherotypia newspaper and zougla.gr news website about 50 minutes before the explosion, which occurred at 10.40 a.m.
About 200 people were evacuated from The Mall in Maroussi. Two guards suffered minor injuries during the blast.
The Mall reopened to customers on Sunday night.
The Council of State, Greece’s highest administrative court, is expected to rule in the next few weeks on whether The Mall, whose retail space covers some 60,000 square meters, was built legally.