BERLIN (Reuters) - German authorities said on Friday they suspected radical Islamists of being behind an explosive device found in an abandoned travel bag in Bonn railway station earlier this week.

Police had previously said they did not know whether the planned attack had a political motive. They said the device could have caused a "large and dangerous fireball" if it had detonated and that they were looking for two suspects.

"There are now sufficient indications that we are dealing with an attempted bomb attack by a terrorist organization of a radical Islamist nature," the federal prosecutors' office said in a statement announcing it would take over the investigation.

"There are credible signs that the suspect (who deposited the bag) has connections in radical Islamist circles," it added.

The statement did not explain why they suspected a radical Islamist connection.

Previously the state prosecutors office in Bonn, in western Germany, had been investigating the case.

German Interior Minister Hans-Peter Friedrich said federal and state authorities would work closely together on the case.

"This affair supports our assessment that Germany is in the crosshairs of jihadist terrorism. The security services are making every effort to overcome this threat," Friedrich said.

The bag found in Bonn station on Monday contained a metal pipe filled with ammonium nitrate, four propane gas cartridges, batteries and an alarm clock.

"Further investigations are needed to discover why the explosive charge did not detonate," the federal prosecutors' office said.

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

German authorities have recently stepped up their surveillance of Salafist groups who espouse a radical version of Islam amid fears they are plotting against the German state.

Germany is home to roughly 4,000 Salafists. They are only a tiny proportion of the total Muslim population of about four million.

(Reporting by Gareth Jones; Editing by Myra MacDonald)