(CBS/AP) The U.S. Embassy in Athens came under fire early Friday from a rocket that exploded inside the modern glass-front building but caused no injuries in an attack police suspect was the work of Greek leftists.

The rocket struck the third floor just missing the embassy emblem. Investigators found the device they believe was used to fire the shell at a nearby construction site, CBS News correspondent Susan Roberts reports.

While police and emergency crews rushed to the scene, traffic across parts of central Athens came to a standstill for more than three hours.

The U.S. ambassador to Greece said the attack came without warning.

"There can be no justification for such a senseless act of violence," said U.S. Ambassador Charles Ries.

Greece's Public Order Minister said police were examining the authenticity of anonymous phone calls to a private security company claiming responsibility on behalf of Revolutionary Struggle, a militant left-wing group.

"It is very likely that this is the work of a domestic group," Minister Vyron Polydoras said. "We believe this effort to revive terrorism is deplorable and will not succeed."

Revolutionary Struggle claimed responsibility for a May 2006 bomb attack on Culture Minister Giorgos Voulgarakis, in which nobody was hurt.

U.S.-owned banks and companies have often been targeted in small bomb attacks by groups in Greece. But Friday's incident was the most serious since the 2002 break-up of the far-left November 17 group, which was blamed for several attacks against foreign diplomats and military personnel, including the assassination of a CIA station chief in Athens.

Polydoras said Greece "strongly condemns" the attack.

"We believe it is a symbolic act," he said. "It is an attempt to disrupt our country's international relations."

Police cordoned off streets around the heavily guarded building after the explosion, stopping traffic in much of central Athens for more than three hours. Emergency services scrambled to the embassy building, a frequent destination for protest groups.

Investigators were examining what they believed was the device used to fire the rocket shell from a construction site near the embassy.

"This is an act of terrorism," Police Chief Asimakis Golfis said. "There was a shell that exploded in the toilets of the building ... It was fired from street level."

Ambassador Ries said the building was not occupied at the time and the damage was minimal. The embassy is now a crime scene and will remain closed until further notice, he said.

"We're treating it as a very serious attack," Ries said.

Authorities were searching nearby apartment buildings and a nearby hospital for evidence.

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