THESSALONIKI, Greece (AP) — A bomb explosion targeted the home of the president of the Greek association of prison guards early Saturday morning in the country’s north.

The guard, named Konstantinos Varsamis on the association’s website, was left unharmed after about 3 kilograms (about 6.6 pounds) of explosives went off at 2:10 a.m. local time, police said. Two people suffered minor injuries from shattered glass.

The explosives were placed outside his apartment building’s front door in Sykies, a suburb in the northern city of Thessaloniki, where he lives on the first floor, according to authorities. The blast damaged three apartment buildings, shattering their windows and collapsing a shared wall, according to an Associated Press reporter on-site.

“I woke up because of the very loud bang of the explosion,” Tzetno Kelo, 52, who lives in an adjacent apartment building, said. “Shattered glass from a window fell on my bed and I was covered in blood.” He was treated at a hospital before being discharged

Police said a witness saw a man walking in the densely built street shortly before the explosion.

Varsamis has worked for many years at Diavata prison, west of Thessaloniki, known for housing many criminal gang members as well as convicted terrorists.

Two police officers said they are focusing their efforts on criminal gangs rather than terror groups. Both spoke on condition of anonymity because they are not supposed to comment on an ongoing investigation. The police have already deposed Varsamis, they said.

The police’s Organized Crime Unit is leading the investigation.