Africa's oldest synagogue was badly damaged by fire
Speaking on German television, Mr Schily said there were "more and more indications" that the blast was a deliberate act.
Eleven German tourists, including a 15-year-old girl who died of injuries on Thursday, were among the victims.
"We are considering all possibilities, but those that we must consider include al-Qaeda structures," Mr Schily said, suggesting for the first time a link to Osama Bin Laden's group.
He announced plans to visit Tunisia at the weekend to discuss the explosion with President Zine El Abidine Ben Ali. He will visit the damaged Ghriba synagogue in the Tunisian resort of Djerba on Monday.
German prosecutors believe a fuel truck parked in front of the synagogue was deliberately detonated by its driver.
Driver suspected
Mr Schily said Germany had "not only eyewitnesses and telephone taps but also material evidence" that the explosion of the fuel tanker was not an accident.
The federal prosecutor's office said it was assuming that the driver was the attacker. He is believed to have died in the blast.
The driver is said to have lived in the French city of Lyon. French and German officials identified him as 24-year-old Nizar Ben Mohamed Nasr Nawar.
Tunisia had insisted the explosion was an accident, but officials have now said they believe the driver of the lorry to be a "suspect".
'Al-Qaeda' letter
French police sources say the driver's name is identical to that published in a letter sent to the London-based Arabic newspaper Al-Quds Al-Arabi.
The writer of the letter admitted responsibility for the explosion on behalf of al-Qaeda.
Earlier this week German police released without charge a man arrested in connection with the explosion.
"The questioning and the examination of elements assembled by the investigators did not allow us to establish any evidence against him," the prosecutor's office said in a statement.