German authorities sent scores of police to secure the nation's railways on Wednesday after a series of firebombs disrupted train traffic for thousands of passengers.
Authorities have detected a total of 15 firebombs in seven different locations since Monday and hundreds of trains were delayed due to partial shut-downs, but no one has been injured in any of the arson attacks.
The attacks "are criminal, terrorist acts," German Transport Minister Peter Ramsauer said.
The Federal Prosecutors' Office, responsible for terrorism-related crimes, announced it was taking over the investigation from local authorities, conducting it on suspicion "of anti-constitutional sabotage."
"I have ordered police to be reinforced, with uniformed and plainclothes officers," Interior Minister Hans-Peter Friedrich said in a televised statement. "We will do everything to arrest the perpetrators."
One firebomb went off midday Wednesday on a train track in the western area of Staaken in Berlin, police spokesman Michael Gassen said. The device caught fire but didn't explode, he said. Two other firebombs failed to explode near train lines in Berlin's south and center.
The other four arson attempts occurred Monday and Tuesday, with one of the devices exploding in a cable shaft along a high speed train link between Berlin and Hamburg, but causing no casualties.
A previously unknown leftist group has claimed responsibility for one of firebombs, saying it hopes to cause widespread disruption and that it opposes Germany's roles in the Afghanistan war.
Police assume all the devices planted at the same time, possibly Sunday. They believe some may have failed to explode thanks to persistent rain in the region since the weekend.
Officials said it was possible that more firebombs were still hidden.
German railway operator Deutsche Bahn promised to pay a euro100,000 ($136,000) reward for hints leading to the perpetrators.
A group calling itself the Hekla Reception Committee Initiative for more Eruptions in Society claimed responsibility for one of the first attacks, saying in an online posting. The group's name is an apparent reference to Iceland's Hekla volcano.
Germany is a large contributor to the NATO-led International Security Assistance Force in Afghanistan, with about 5,000 soldiers stationed in the north of the country.
In recent months, there also have been several arson attacks in Berlin blamed on unidentified leftist extremists.
One of them targeted a utility shaft at an important railway hub in the city's east in May, causing massive disruption that left thousands of passengers stranded.
More than 100 cars also have been torched in Berlin over the past several months, some of them blamed on vandalism motivated by animosity against the rich.