German prosecutors have launched an investigation into why a Moroccan man convicted of assisting the 9/11 suicide plotters was allowed to leave Germany with an envelope containing thousands of euros in cash.

Mounir el Motassadeq, who was deported from Germany in October after 15 years in prison for his role in the plot as a member of a terrorist organisation, was handed the payment of €7,000 – the wages accumulated for the prison work he did, plus a monthly allowance of around €30 – before leaving the country.

It is forbidden to pay the money from a “prisons account” to terrorists or terror suspects because their assets are meant to be frozen and inaccessible.

A spokeswoman for the Hamburg prosecution service confirmed to Der Spiegel magazine that it had launched an investigation into a suspected violation of paragraph 18 of Germany’s foreign trade and payments law after receiving a complaint from the central bank, which is meant to approve exemptions from the ban but was apparently not consulted in this instance.

The bank alerted authorities to the oversight eight days after Motassadeq’s departure, Der Spiegel said.

Motassadeq was a student friend of the suicide pilots Mohamed Atta, Marwan al-Shehhi and Ziad Jarrah, and was part of the “Hamburg cell”, after the northern port city in which they lived. Motassadeq maintained his innocence during his 2006 trial, which drew worldwide attention, and heard victim impact statements from relatives of those who died in the 9/11 attacks.

He claimed he had merely been a close friend of the men, but the court decided he had helped the cell with the logistics of planning their attacks, including keeping up rental payments once they had left for the US, in order to maintain the appearance that they were planning to return.

He is now believed to be living with relatives in Marrakesh and has refused to talk to reporters who have discovered his whereabouts.

The case is highly embarrassing for authorities in Hamburg who choreographed the departure on 15 October of the high-profile prisoner down to the last detail, having apparently planned it over several weeks. He was brought by masked special police to the airport, and in photographs published in the German media was shown smiling from his seat on the plane flanked by two police officers and surrounded by ordinary passengers on a Royal Air Maroc flight to Casablanca.

Hamburg’s interior minister, Andy Grote, had said his departure was a sign that “we have drawn a line under this chapter”.