German prosecutors on Tuesday questioned and then released a man detained in connection with a truck explosion that killed 15 people at a Tunisian synagogue and said indications of a terrorist attack were growing, just as a group linked to Usama bin Laden claimed responsibility.

U.S. authorities were investigating the claim, a government official in Washington said. If verified, it would make the blast at the Ghirba synagogue on the resort island of Djerba the first terrorist attack by bin Laden's Al Qaeda network since the Sept. 11 attacks in the United States.

German officials took a suspect into custody Monday in the city of Duisburg after receiving a tip from Tunisian authorities that the suspected attacker, identified by Tunisian officials as the driver of the gas-laden truck, had spoken by phone with the suspect hours before the blast last Thursday.

But authorities freed the unidentified suspect after questioning and gave no indication of the man's role, if any, in the case. A statement from Germany's Federal Prosecutor's Office said that searches of the suspect's apartment and those of several associates also turned up no compelling evidence.

Still, the statement said: "Indications that the event of April 11 was a terror attack have strengthened further. Further intensive investigation is still needed to clear up the exact sequence of events."

German officials, including Interior Minister Otto Schily, declined to comment on a report in the news magazine Stern suggesting the man had links to Al Qaeda. They also would not comment on whether investigators had found ties between the Djerba explosion and two terrorist cells believed to have operated in Germany.

"We can't judge that at the moment," said Gerhard Schlemmer, a federal police spokesman.

Five Algerians linked to Al Qaeda and charged with plotting to blow up a French market went on trial Tuesday in Frankfurt. U.S. authorities have said that the Sept. 11 terrorist attacks were orchestrated by a terrorist cell that operated in Hamburg, Germany.

On Tuesday, two London-based Arab newspapers reported responsibility claims for the Tunisian synagogue attack from a group calling itself the Islamic Army for the Liberation of the Holy Sites — the same name used by a group that claimed the 1998 bombings of the U.S. embassies in East Africa.

The group was believed to be the military wing of a militant coalition dominated by Al Qaeda and the Egyptian Islamic Jihad. Tuesday's report said the group acted in retaliation for "Israeli crimes" against Palestinians.

Prosecutors said they were evaluating the responsibility claims but Schily urged caution. "Whether such declarations are really connected with this terrible act is always an open question."

In Washington, a U.S. official who spoke on condition of anonymity said the explosion "certainly looks as though it's as an act of terrorism. We're looking into the links [to Al Qaeda]."

Tunisia had insisted the blast was a "tragic accident," sparked when a tanker truck accidentally struck the synagogue wall.

But a Tunisian government statement Tuesday said investigators were not ruling out anything "including possible links that the suspect could have had with elements established in Germany," the official TAP news agency reported.

The statement said the driver had lived with his family in Lyon, France, but didn't identify him further. It also referred to a wave of anti-Semitic attacks in France and other European countries following the recent Israeli offensive in Palestinian areas, which has outraged Arabs around the world.