Greece's anti-terrorist police launched a major manhunt on Wednesday after an officer guarding a witness whose testimony led to the jailing of left-wing extremists was gunned down on duty.
Nektarios Savas, 41, was shot dead around daybreak in Athens while sitting in his unmarked vehicle outside the witness's home when three unknown assailants on motorbikes opened fire.
He was hit by around 15 bullets and declared dead in hospital.
Savas, a married father of one child, was attacked at around 6:20 am (0320 GMT), outside the home of Sofia Kyriakidou, who was a state witness in the 2004 trial of four followers of the extremist People's Revolutionary Struggle (ELA).
The four were sentenced to 25 years in jail but later released, mostly for health reasons.
The ELA, Greece's most notorious extremist outfit after the dreaded November 17 group, was formed in 1974 after the collapse of the military junta and specialised in striking police, banks, government offices and US interest targets. It ceased its activities in 1995 after some 250 attacks.
But other extremist groups have emerged since then and Wednesday's killing was "the third attack recently against police officers," said police spokesman Panagiotis Stathis.
He added that so far no one has claimed responsibility for Savas's murder.
Police have become a particular target of extremist groups since last December when violent riots broke out across Greece in protest over the killing of an Athens teenager by a police bullet.
Two local radical groups have been responsible for some recent violence, the Revolutionary Struggle (EA) and the Revolutionary Sect, which just surfaced in February.
The EA group is listed as a terrorist organisation by the European Union and the United States and has been blamed to date for some 11 attacks in Athens.
It is known for a rocket-propelled grenade attack on the US embassy in Athens in January 2007 as well as the two bomb strikes against US-based banking group Citibank, one of them aiming to blow up the bank's Greek headquarters.
The far-left group has also twice ambushed police with assault weapons and nearly killed a young officer outside a ministry building.
The attacks have forced police back to the drawing board less than a decade after the 2002 dismantling of Greece's deadliest far-left organisation November 17, blamed for 23 assassinations between 1975 to 2000, including two police officers.
"Setting fire to a car is one thing, but here we are dealing with something more worrying -- pre-planned operations by organised groups," police spokesman Stathis said earlier this year.
Back in February the previously unknown Revolutionary Sect strafed the Korydallos police station in the Athens suburbs. No was injured but the group issued a text stating that its goal was to blindly "execute" Greek police.
A prominent criminologist, Yiannis Panoussis, has said that these extremist groups "feel legitimised by the December crisis" after the teen's murder and have "crossed a line into more stringent and blind violence."