On 3 May 2002, German Security authorities issued a warning they that Islamic terrorists could be planning to hijack passenger ships cruising in European waters, but said they had doubts about the information’s authenticity. The Federal Criminal Office said it was investigating the information and had alerted authorities in other European countries. The statement added that there was no detailed information on the alleged targets, but said a suicide attack could possibly accompany such an action. German authorities had stepped up efforts to track down Islamic militants since the Sept. 11 attacks on the United States, which were planned by a Hamburg-based terror cell. The alert concerning passenger ships was presumably based on the ongoing interrogation of nine men arrested on 23 April 2002 in a sweep against a suspected Islamic terrorist cell in Germany.