Gunmen sprayed Athens riot police with automatic weapons fire early Monday, seriously wounding a policeman in an escalation of violence that broke out after the fatal police shooting of a teenager last month.

Police said one of the weapons fired by the attackers had been used previously by the extremist group Revolutionary Struggle, which claimed responsibility for a 2007 attack on the U.S. Embassy in Athens. No one was hurt in the embassy attack but the U.S. State Department offered a $1 million reward for information leading to the capture of the attackers.

Authorities said more than 30 shots from a 9 mm weapon and an automatic rifle were fired early Monday at three riot policemen standing guard outside the Culture Ministry. Ballistics tests showed the 9 mm weapon had also been used in an April 30, 2007, police station attack that was claimed by the Revolutionary Struggle extremist group.

• Click here for photos from the scene of the attack.

The group had also claimed the January 2007 attack on the American Embassy.

Greek police chief Lt. Gen. Vassilis Tsiatouras said the Kalashnikov-type rifle used Monday had also been used before — in a Dec. 23 attack on a police bus as it passed a university campus. No one on the bus was injured.

Masked protesters have frequently attacked police with gasoline bombs and rocks in the outburst of rage that followed the fatal shooting of a 15-year-old boy on Dec. 6. The riots were the worst the country has seen in decades. But none of those attacks had caused serious injuries.

Monday's shooting severely wounded 21-year-old policeman Diamandis Matzounis. The hospital where he was being treated listed him in critical but stable condition.

A police officers' association said the riots last month had cultivated an atmosphere of hatred against police.

Several shadowy left-wing Greek extremist groups have been active in Greece in recent years. Their attacks usually are small bombings that rarely cause deaths. Authorities cracked down on the groups before the Athens 2004 Olympics.

The country's deadliest terrorist organization, the November 17 group, was blamed for the killings of 23 people between 1975 and 2000. It was broken up in 2002.

The U.S. State Department's Rewards for Justice program said on its Web site that Revolutionary Struggle "is believed to be an offshoot" of November 17.