Although the car bomb planted at the offices on Feb. 18 failed to detonate, police said it contained enough explosives to destroy the four-story building. Monday's attack on a Citibank branch in a northwestern district of Athens caused damage but no injuries.

In a statement sent to a weekly newspaper, Revolutionary Struggle claims responsibility for both attacks, verbally attacks the media and journalists and explains "the ways in which it will continue its activities," the newspaper said Wednesday.

To Pontiki, a satirical weekly, often receives Revolutionary Struggle's declarations. It said it would publish the full statement Thursday.

Revolutionary Struggle first appeared in 2003 and is best known for firing a rocket-propelled grenade into the U.S. Embassy in 2007. In December, it seriously wounded a riot policeman in a gun and grenade attack outside the Culture Ministry in what it later said was a response to the fatal police shooting of a teenager in December.

The boy's death sparked major riots across the country, and has been followed by an escalation of violence by domestic militant organizations.

An apparently new group, which some believe could be a Revolutionary Struggle splinter, emerged in early February with a gun and grenade attack against an Athens police station in response to the teenager's killing. It later launched a similar attack outside a Greek TV channel. Neither caused any injuries.

The new group, Sect of Revolutionaries, has threatened to continue targeting police and journalists.

Greece faced targeted attacks by domestic terrorists for decades, mostly by groups seeking to portray themselves as urban revolutionaries championing the poor and oppressed. Authorities believed the problem had diminished after the arrest of several members of the country's deadliest group, November 17, after a botched bombing in 2002.

But the frequency and scale of recent attacks has led some experts to voice concern that the current groups are motivated less by ideology than a desire to kill.

In its annual report on terrorism last year, the U.S. State Department said it believed Revolutionary Struggle could be linked to November 17, which killed 23 people between 1975 and 2002.

"Revolutionary Struggle is a radical leftist group, which aligned itself with the ideology of N17 and may have incorporated some previous members of N17," the report said.