ERLIN, Dec. 23 — A German man under investigation for links to top figures of Al Qaeda slipped out of the country last month, withdrawing his four children from school, terminating his lease and obtaining visas for Saudi Arabia without attracting any attention from the police, according to German officials.
Christian Ganczarski, 36, a Polish immigrant who until recently lived in the western German city of Mülheim, had been under investigation since the German police overheard a telephone call from Nizar Nawar, shortly before Mr. Nawar detonated a bomb on April 11 in front of the Ghriba Synagogue on the Tunisian island of Djerba. The blast killed 21 people, including 14 German tourists.
Prosecutors overseeing the investigation say that under German law, the evidence tying Mr. Ganczarski to the bombing and his own confession of recent contact with Qaeda leaders were insufficient to keep him under constant surveillance or to prevent him from traveling. They say those limitations are the consequence of a Constitution devised to prevent the reoccurrence of the country's totalitarian past.
The case has caused concern among officials in France and Tunisia involved in an investigation into the Djerba bombing and illustrates the complexities of fighting a global network like Al Qaeda.
Last week, the Tunisian justice minister complained openly about Mr. Ganczarski's departure. "Investigations into the attack on Djerba have moved forward very well, and I hope that the flight from Germany of an accomplice of the suspected perpetrator of the attack will not hamper inquiries," the minister, Bechir Tekkari, told Agence France-Presse.
In a recent interview a high-ranking French official, who insisted on anonymity, expressed frustration that Mr. Ganczarski had not been detained. Under French law, the official said, "he would have been."
Mr. Ganczarski is a figure who German prosecutors say may have been able to provide unique knowledge of Qaeda cells. Under interrogation, he has admitted to traveling five times to Qaeda camps in Afghanistan, including once about a month after the Sept. 11 attacks.
"This suspect stands out from all others because he has had contact with the inner circles of Al Qaeda," said Kay Nehm, Germany's chief federal prosecutor, in a recent interview. "We do not find such a witness every day. He is someone who knows a lot."
He is also a European Muslim convert, which might pose some concern to investigators. "The blond-haired, blue-eyed Al Qaeda terrorist is an investigator's nightmare because he does not fit the typical profile," Col. Nick Pratt, a professor at the George C. Marshall European Center for Security Studies in Garmisch-Partenkirchen, Germany, said in an interview.
Until recently, Mr. Ganczarski, who was unemployed, lived in Mülheim, near the industry of the Ruhr Valley. As a child, he immigrated to Germany from Poland under laws providing for the return of ethnic Germans from Eastern Europe.
In the mid-1990's, Mr. Ganczarski converted to Islam. The police believe that he was recruited by Al Qaeda in nearby Duisburg. The suspected recruiter, Elfatih Musa Ali, also under investigation in connection with the Tunisian blast, left Germany for Sudan in May, according to the German weekly Der Spiegel.
In the weeks following the bombing on Djerba, Mr. Ganczarski was questioned on a number of occasions. The German police had been monitoring him because he had been seen in the company of known extremists at a mosque in Duisburg.
However, they did not realize the potential significance of the telephone call until after the bombing. Although Mr. Ganczarski admitted to having met Mr. Nawar while in Asia, he denied any connection to his plot, and the police, who have released parts of the transcript, say his involvement was impossible to prove.
"Don't forget to remember me in your prayers," Mr. Nawar told Mr. Ganczarski, according to the transcript.
"God willing," replied Mr. Ganczarski. "Do you need anything?"
"No thanks," came the reply from Mr. Nawar. "I need your blessing."
"God willing, O.K.," Mr. Ganczarski said.
Shortly before the blast, Mr. Nawar had also called Khalid Shaikh Mohammed, who is widely believed to be one of the chief planners of the Sept. 11 attacks.
The deaths of the German tourists on Djerba prompted German legislators to revamp terrorism laws intended to combat domestic terrorism during the 1970's and 80's. But Mr. Nehm said he could not use new laws, which allow the prosecution of members of foreign terrorist organizations, in Mr. Ganczarski's case because they were passed after the Djerba attack and were not made retroactive.
The failure to monitor Mr. Ganczarski occurred after he had confessed to the police that he had been in Afghanistan five times and that he had met on occasions with Mr. Mohammed and once with Osama bin Laden.
In their parallel investigation, the French police recently arrested a number of suspects, including Mr. Nawar's brother Walid Nawar, in connection with the Djerba attack. They say Walid Nawar bought the satellite phone that Nizar Nawar used to call Germany and Pakistan.
Before the Djerba attack, investigators say, Nizar Nawar traveled to diverse parts of the world including Afghanistan and Montreal. Investigators say a Qaeda cell in Montreal, led by Mohambedou Ould Slahi, conceived the failed 1999 plot to bomb Los Angeles International Airport. Mr. Slahi, reportedly related by marriage to Mr. bin Laden and now in American custody at Guantánamo Bay, Cuba, was often seen in the late 1990's with Mr. Ganczarski in Duisburg.