The federal prosecutor's office in Karlsruhe said El Motassadeq was charged in a Hamburg superior court in "connection with his connection to the terror attacks on the United States," but did not announce specific charges.
El Motassadeq, 28, was picked up Nov. 28 at his Hamburg apartment on suspicions he had "intensive contacts" with the Sept. 11 hijackers who had been living in Hamburg, including alleged ringleader Mohamed Atta, Marwan Al-Shehhi and Ziad Jarrah.
Prosecutors in the past accused him of running Al-Shehhi's bank account, using it to finance the hijacker's stay in the United States and his flight training school in Florida.
El Motassadeq's name appeared on a U.S. list of 370 individuals and organizations with suspected links to the Sept. 11 attacks that Finnish financial authorities made public in October. When contacted at that time by The Associated Press, El Motassadeq angrily denied any involvement.
"All of this is false, I have nothing to do with this thing," he said before hanging up.
Germany has issued international arrest warrants for several other suspects, but El Motassadeq is the only person who has been arrested in the country in connection with the Sept. 11 attacks.
El Motassadeq has been an electrical engineering student at Hamburg's Technical University since 1995 — the same school where Atta, 33, and Al-Shehhi, 23, studied before leaving Germany last year for the United States.
In an October television interview, El Motassadeq admitted having power of attorney on Al-Shehhi's account but said he never made any transfers. He said he knew Atta and some of the other hijackers, and visited the apartment where Atta, Al-Shehhi and others lived.
El Motassadeq said he had signed Atta's will, as did others at the al-Quds mosque.
From 1996 to 1998, El Motassadeq worked as a cleaner at the Hamburg airport and had access to secure areas and aircraft. He passed a routine security check in 1996 before starting the job.
After his arrest, it was also uncovered that El Motassadeq visited a nuclear plant near Hamburg in May 2001, as part of a university trip. The school said, however, that El Motassadeq had to be cajoled into coming and did not take any notes on the trip, which was part of a sightseeing tour of the city of Stade.