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KARLSRUHE, Germany - Germany's highest appeals court ruled on Thursday that a Moroccan friend of the September 11 hijackers was guilty of abetting mass murder and must return to court to receive a new, harsher sentence.

Mounir El Motassadeq, a member of a group of radical Arab students in Hamburg who organised the 2001 attacks, was convicted last year of belonging to a terrorist organisation and given a 7-year jail sentence.

But the court cleared him of abetting mass murder, saying Motassadeq was only a lower-tier member of a group led by Mohamed Atta, who flew the first hijacked plane into New York's World Trade Centre.

Judge Klaus Tolksdorf overturned that verdict on Thursday, saying Motassadeq was guilty of aiding in the deaths of 246 passengers and crew members, excluding the hijackers, who died on the four planes that crashed on September 11.

Two planes were steered into the World Trade Centre, another hit the Pentagon and a fourth crashed in a field in Pennsylvania.

Motassadeq must now return to a court in Hamburg, where he has been tried on two previous occasions, to receive a new sentence. He faces up to 15 years in prison.

"He provided assistance for the murderous acts of the terror pilots," Tolksdorf told the court. "By assuming organisational tasks, he aided and abetted the attacks."

Motassadeq is one of only a handful of men to have been tried in connection with the September 11 attacks that killed nearly 3,000 people.

Zacarias Moussaoui, a French citizen of Moroccan descent, was sentenced to life in prison in May. He is the only person convicted in a US court in relation to the hijackings.

"I think all the family members are going to be incredibly happy," Dominic Puopolo Jr., a 40-year old American whose mother was on the American Airlines flight piloted by Atta, told Reuters after the Motassadeq decision. "We couldn't have asked for a better result."

Motassadeq's lawyers have insisted he knew nothing about the September 11 plot. They appealed last year's conviction, saying there was also no proof he belonged to a terrorist organisation.

The prosecutors appealed as well, arguing the court should have found Motassadeq guilty of the more serious charge because he knew the hijackers intended to use planes for mass murder.

"The justice system has proven it can handle unusual criminal offences like this," Gerhard Altwater, a federal prosecutor, said.

The September 11 attacks were planned and organised by a Hamburg al Qaeda cell Motassadeq was linked to. In his second trial, the judges said Motassadeq knew too little of the cell's plans to convict him of abetting mass murder.