An explosive device found in an abandoned travel bag in a railway station in the west German city of Bonn yesterday could have caused a "large and dangerous fireball" if had detonated, German police said today.

Police officials in Cologne in charge of the investigation said they were searching for two men suspected of leaving a blue travel bag on a platform in nearby Bonn. Two suspects initially detained by police were released on Tuesday.

Officials said a restaurant surveillance camera recorded images of a man carrying a bag similar to the one packed with explosives. Police said the bag contained a metal pipe filled with ammonium nitrate, four propane gas cartridges, three batteries and an alarm clock.

There has been much confusion about the bomb since it was discovered on Monday. It is unclear how the attack was foiled and why the bag was left at the rail station.

"The propane gas cartridges would have caused a very large and dangerous fireball," said Norbert Wagner, the head of the investigation for the Cologne police. "The chemicals were highly explosive."

Wagner denied German media reports that the bomb would have been as destructive as the 10 explosive devices used on four commuter trains in the Islamist militant attack in Madrid in 2004 that killed 191 people.

Germany narrowly missed an attack in 2006 when two suitcase bombs left on commuter trains in Cologne failed to explode.

"It (Bonn bomb) was an extremely dangerous explosive device," said Rainer Griesbaum, a federal prosecutor in charge of terror cases. But Griesbaum said it is not yet clear whether those responsible for the planned attack in Bonn were politically motivated or not. He said initial indications that it was a terror attack could not be substantiated.

"To sum it all up, everything is possible."

The bag was reported to railway station officials at midday on Monday. Authorities then shut the station down. The bag was then opened with the help of a robot.