(CNN) -- German authorities suspect Islamist extremists were responsible for planting an explosive device Monday on a track at the main railway station in Bonn, a German intelligence official tells CNN.
The explosives were found after a 14-year-old reported the bag to police, according to the official, who said the device was "not sophisticated" in design.
The official said whoever left the bag remains at large. Initially, German police arrested two Bonn residents soon after recovering the explosive components, the official said. The official identified them as Omar D., who's long been on German security services' radar because of his alleged links to Islamist extremists, and Abdirazak B.
Both, however, were released without charge after just a few hours in custody. The official said authorities have not ruled out Omar D. as a suspect but do not have enough evidence to hold him.
On Wednesday German police released a composite sketch of the suspected perpetrator based on a description from the 14-year-old. A German official told CNN the sketch describes a tall, thin, dark-skinned man in his early 30s.
Several German newspapers Tuesday reported that German police had arrested a German citizen of Somali origin, identified as Omar D.
Yassin Musharbash, an investigative reporter at Die Zeit, told CNN that security services had been aware of Omar D. for several years because he is believed to have links to Islamist extremists in the Bonn area.
Omar D. and Abdirazak B., a Somali associate from Bonn, were arrested by German police in 2008 at the Cologne Bonn Airport as they prepared to travel to Uganda, according to a briefing prepared by the London based-International Centre for the Study of Radicalisation. It stated that German authorities suspected the duo planned on waging jihad in Somalia or Pakistan, but they were soon released.
Der Spiegel said the bag left at the Bonn station contained butane gas, ammonium nitrate, a metal pipe, an alarm clock and batteries. Die Zeit's Musharbash told CNN that according to investigators, an initial assessment indicated that the device was rudimentary in design. He said it will probably take several days to judge how dangerous the device was.
He said investigators had yet to find a detonator among the recovered components, calling into question whether the device would ever have worked.
Based on a preliminary investigation, Musharbash said it should not be ruled out that a bomb disposal team that responded Monday unwittingly destroyed the detonator. Investigators are said to be looking into this possibility, but this seemed like only one possible scenario, he said. A German intelligence official told CNN on Wednesday that no detonator had yet been found but that a water cannon used to make the device safe may have destroyed the detonator.
Musharbash told CNN it was not yet clear if the bombing attempt was merely the work of homegrown "amateurs" or had connections to overseas terrorist groups.
In October 2011, a militant left-wing group claimed responsibility for placing several incendiary devices on railway lines in Berlin. The German intelligence official said that while it could not be completely ruled out that a similar group left the explosives at the Bonn railway station, the nature of the device found in Bonn was significantly different from the devices planted by the left-wing group in Berlin. Whereas the Berlin devices had been built to disrupt train services, the Bonn device, though it may never have been viable, appeared to have been built to "blow people up" as they boarded or exited trains, the official told CNN.
Terrorists inspired by or affiliated with al Qaeda have a track record of targeting rail lines in Europe.
Several weeks after the Madrid bombings in 2004, the cell that carried out the attack attempted to blow up a bridge on the Madrid-Seville high-speed train line. The plot was abandoned after the terrorists realized they had been spotted by nearby workers, who alerted police, a Spanish official told CNN. The official added that hundreds could have been killed given the quantity of explosives left at the scene.
In 2006, two pro-al Qaeda Islamists placed bombs on two commuter trains leaving the Cologne area, but the attack failed when the devices didn't detonate. Authorities later said the devices could have killed up to 70 people if they had been properly constructed.
Intelligence materials recovered from al Qaeda leader Osama bin Laden's compound in Abbottabad, Pakistan, revealed that in 2010, al Qaeda discussed an idea to derail a train in the United States by obstructing tracks, several U.S. officials told CNN.
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