Greek police have been the target of protests and attacks
A left-wing militant group has claimed responsibility for shooting and seriously injuring a 21-year-old police officer last week, police say.
In a statement sent to a newspaper, Revolutionary Struggle also said it was behind a gun attack on a police bus on 23 December, in which no-one was hurt.
Revolutionary Struggle has carried out attacks before, including firing a rocket at the US embassy in Athens.
Analysts say they fear Greece's recent unrest could fuel domestic terrorism.
The attacks follow Greece's worst riots in decades, sparked by the fatal police shooting of a teenager in Athens in early December.
Link to November 17
Police said Revolutionary Struggle had sent a pamphlet to the weekly Pontiki newspaper, claiming responsibility for the 23 December attack on the bus, the 5 January shooting of a policeman, and a failed bomb attack on Shell's headquarters in Athens.
"The anti-terrorist squad has gone to the Pontiki weekly's offices and is investigating," police spokesman Panagiotis Stathis told Reuters.
It was not clear whether Revolutionary Struggle had given any reason for their attacks.
The anti-capitalist group is seen by some as Greece's most dangerous militant threat, following the break-up of the November 17 organisation in 2002.
The US state department says Revolutionary Struggle has "aligned itself with the ideology of N17" and may have incorporated some of its former members.
It believes the group is behind several attacks, including the 2004 murder of a Greek guard outside the British defence attache's residence, and a bomb attack on the Greek culture minister in 2006.