German investigators said Friday that two bombs found last month on regional trains were likely linked to terrorists, but were cautious about drawing conclusions from Lebanon-related material found with the devices.

The bombs, made with gas canisters, were found July 31 on trains in Dortmund and Koblenz. Joerg Ziercke, the head of Germany's Federal Crime Office, said they apparently were supposed to explode 10 minutes before the trains' arrival at the stations.

"The scenario could have looked like this: two simultaneous bomb explosions in regional trains, a fireball ... burned-out train carriages, an undetermined number of injuries and possibly deaths, and possibly derailed trains too," Ziercke told reporters.

He noted that, unlike commuter trains attacked in Madrid in 2004 and subway trains bombed in London last year, the trains involved were relatively empty and not traveling at peak time, suggesting the intention was not to cause mass fatalities.

Ziercke said investigators did not believe they were dealing with an attempt blackmail of the train operator, Deutsche Bahn, and that "a possible terrorist background is more probable, but that comes with reservations."

Investigators said a ripped-up scrap of paper with Arabic script listing groceries and phone numbers in Lebanon had been found in clothing surrounding one gas bottle, while small bags of starch, also from Lebanon and available in Germany, also were found.

"One can imagine — I stress imagine — that those responsible wanted to send a signal regarding the conflict in the Middle East," Ziercke said. However, he noted that the scrap would have been destroyed had the bomb exploded.

Asked whether an Islamic fundamentalist background was suspected, he did not reply directly but said: "You can assume that, in such a situation, we are exhausting all possibilities."

He said the apparent attackers did not plan suicide bombings, given that they left the trains before the planned detonations. The bombs were supposed to be triggered by alarm clocks.

Rainer Griesbaum, representing German federal prosecutors, said they were investigating for possible charges of belonging to or founding a German-based terrorist group, attempted murder and attempting to bring about an explosion.

Ziercke said there had still been no claim of responsibility, and asked for the public's help in finding the people who planted the devices.

Investigators released a video showing two suspects, both men, coming up an escalator with heavy wheeled luggage in Cologne station, which both trains passed through. They were believed to be between 20 and 30 years old.

Authorities offered a US$64,300 reward for information leading to the capture of those responsible.

That figure shows "that we are very concerned and have to assume that the danger continues," Ziercke said.