"When he saw the pictures, he admitted that he planted the bomb," Lebanon's acting Interior Minister Ahmad Fatfat told Reuters television.

"There is nothing clear now, but in his computer, we found something that can be related to al Qaeda, some contacts. He denied any contact til now with al Qaeda, but maybe he was working without knowing (it) himself under the umbrella of al Qaeda," he added.

Fatfat was referring to Jihad Hamad, a 20-year-old Lebanese man who turned himself in to authorities in the Lebanese city of Tripoli on Thursday.

Hamad and a 21-year-old Lebanese man named Youssef Mohamad E.H., who was arrested in Germany at the weekend, were caught on security videos dragging suitcases on to trains in Cologne last month.

Suitcases like those in the footage were found packed with propane gas tanks and crude detonating devices on trains in Dortmund and Koblenz. The explosives failed to detonate.

Authorities in Germany and Lebanon on Friday detained two more suspects in connection with the bomb plot.

Germany's federal prosecutors office said police in the southern state of Baden-Wuerttemberg had taken into temporary custody a new suspect and that they had searched a room in a student dormitory in the city of Konstanz.

Lebanese authorities have also arrested a new suspect they believe may have been involved in the plot, a senior Lebanese judicial source said in Beirut.

The Lebanese source said the 24-year-old Lebanese man, with the initials K. H. D., was arrested based on information provided by Hamad.

The detentions bring to four the number of suspects in custody, two in Germany and two in Lebanon. No formal charges have been filed against them.

German prosecutors said they were investigating whether the newly detained suspect was involved in preparing the attacks.

They said he was among acquaintances of Youssef Mohamad E.H., who was apparently preparing to flee the country when German police caught up with him on Saturday.

Germany's federal police chief said it remained unclear whether the suspects could be linked to an international militant network or merely part of a local association. German investigators had gone to Beirut to question Hamad.

Both Hamad and Youssef Mohamad E.H. had been living in Germany and initially fled to Lebanon shortly after the bombs were discovered on July 31, police say. Youssef Mohamad E.H. returned while Hamad remained in Lebanon.