Authorities have arrested six suspects, including a citizen of a former Soviet republic, on suspicion of involvement in the weekend's deadly explosion in this holy Shiite city which killed 13 people, a police official said Thursday.

The unidentified foreigner allegedly admitted he had been trained by Osama bin Laden's terror network, Al Qaeda (search), in a European country, police spokesman Rahman Meshawy said.

Five Iraqis, believed to be linked to terror organizations, were rounded up Wednesday in a separate raid, he added. The men allegedly confessed that there were several Arab nationals, including two from Persian Gulf (search) countries, involved with them in terrorist activities.

The deadly car bomb attack on Sunday at a bus station in Karbala (search), some 80 kilometers (50 miles) south of Baghdad, killed 13 and injured 33. Another car bomb on the same day in Najaf, south of Karbala, killed 54 people.

Meshawy said police have imposed tight security around the city and are searching for other suspects.