Paris - The man who helped plot the 9/11 terrorist attacks and two others went on trial in Paris on Monday for their alleged involvement in the 2002 suicide bombing of a Tunisian synagogue that killed 21 people.

Khalid Sheikh Mohammed, who has admitted being the main architect of the 9/11 attacks, German citizen Christian Ganczarski and Tunisian Walid Nauar, the brother of the suicide bomber, are charged with accessory to murder and face a maximum sentence of life in prison if convicted.

Khalid Sheikh Mohammed, who is currently imprisoned in the US detention facility at Guantanamo Bay, is being tried in absentia.

All three are suspected of having played a part in the planning and execution of the suicide bombing of an ancient synagogue on the Tunisian island resort of Djerba that took the lives of 14 German tourists, five Tunisians and two French citizens on April 11, 2002.

In the attack, the suicide bomber, Nizar Nauar, a native of Tunisia, drove a fuel tanker up to the synagogue and blew it up.

Khalid Sheikh Mohammed was allegedly Nauar's spiritual mentor. Ganczarski - alias Ibrahim ou Abu Mohammed - allegedly helped plan the bombing and spoke with Nizar Nauar by cellphone just hours before it was carried out.

The trial is expected to last one month. (dpa)