Khalid Sheikh Mohammed is being tried in absentia for the 2002 attack
Three men - including the alleged mastermind of the 9/11 attacks - have gone on trial in France for the 2002 bombing of a Tunisian synagogue.
Khalid Sheikh Mohammed, who is being held in the Guantanamo Bay camp, is being tried in absentia for the attack.
German national Christian Ganczarski and Tunisian Walid Nouar are also on trial in Paris. They deny involvement.
Twenty-one people died when a suicide bomber drove a gas-filled tanker into the Djerba synagogue on 11 April 2002.
Al-Qaeda claimed responsibility for the attack.
Kuwaiti-born Khalid Sheikh Mohammed is accused of organising the bombing, which killed 14 German tourists, five Tunisians and two French nationals.
Under French law the death of the two French means a trial can be held in France.
According to court documents, suicide bomber Nizar Nouar called Khalid Sheikh Mohammed and Mr Ganczarski, a convert to Islam who specialised in communications, just before he drove the gas-laden truck into the synagogue.
Christian Ganczarski said he had "nothing to do with the attack"
The calls were allegedly made on a telephone brought into Tunisia by the bomber's brother, the third defendant Walid Nouar.
All three men have been charged with "complicity in attempted murder in relation to a terrorist enterprise". They face 20 years in prison if convicted.
Relatives of the victims were in court on Monday.
"We are hoping for a life sentence... and we think there is sufficient evidence," said Judith-Adam Caumeil, a lawyer for German families.
Christian Ganczarski, a Polish-born German, identified himself to the court in German and insisted on his innocence.
"I had nothing to do with the attack," he said.
The bomber's uncle, Belgacem Nouar, was jailed in 2006 for his role in the attacks.
The trial is due to last until 6 February.