One of the only two people who has been tried and sentenced in connection to the Sept. 11 terrorist attacks is a free man again – and is heading back to his native Morocco.

Mounir el-Motassadeq boarded a flight in Frankfurt on Monday after his conviction more than a decade ago for being a member of a terrorist organization and accessory to the murder of the 246 passengers and crew on the four jetliners used in the attacks.

"It's a good feeling to know that Mr. Motassadeq is out of the country," Hamburg's Interior Minister Andy Grote told The Associated Press.

El-Motassadeq was released shortly before completing his 15-year-sentence on the condition that he agreed to deportation to Morocco.

"This measure will allow us to arrest him immediately should he set foot on German soil again," Frauke Köhler, a Federal Prosecutors Office spokeswoman, told Deutsche Welle.

It’s not clear what awaits the 44-year-old when he lands in Morocco.

The only other person sentenced in relation to 9/11 – co-conspirator Zacarias Moussaoui – has been serving a life sentence without parole in Colorado’s ADX Florence, the same facility that houses Boston bomber Dzhokhar Tsarnaev, Unabomber Ted Kaczynski, and other Al Qaeda operatives, like "shoe bomber" Richard Reid.

Other men involved in the planning of the attacks have been stuck in legal limbo, leaving victims’ families increasingly frustrated, with some saying justice has yet to be served.

In el-Motassadeq’s case, German courts had ruled he was aware Mohammed Atta, Marwan al-Shehhi and Ziad Jarrah – three of the 9/11 hijackers – planned to take over and crash the planes, even though he might not have known specifics of the plot.

They said, according to the Associated Press, that el-Motassadeq helped "watch the attackers' backs and conceal them" by helping them keep up the appearance of being university students paying tuition and rent and transferring money.

El-Motassadeq acknowledged training at an Al Qaeda camp in Afghanistan but had insisted he knew nothing of his friends' plans to attack the U.S.

"I swear by God that I did know the attackers were in America," he had shouted in accented German at a sentencing hearing. "I swear by God that I did not know what they wanted to do."

SEPTEMBER 11 PLOTTERS: WHERE ARE THEY NOW?

Originally arrested in Hamburg in November 2001, el-Motassadeq was convicted in 2003 of membership in a terrorist organization and thousands of counts of accessory to murder — taking into account victims on the ground — becoming the first person convicted anywhere on charges related to Sept. 11. He was sentenced to the maximum 15 years in prison.

However, a German federal court overturned that verdict in 2004, largely because of a lack of evidence from Al Qaeda suspects in U.S. custody, and sent the case back to Hamburg.

After a 2005 retrial, el-Motassadeq was again convicted of membership in a terrorist organization. But he was acquitted of being an accessory to murder after the court ruled it didn't have enough evidence that he knew of the hijackers' plot.

El-Motassadeq was sentenced to seven years in prison at the time, but was freed in early 2006 until his appeal could be heard, according to the Associated Pres.

Later that year, the federal court reversed the Hamburg court's acquittal of el-Motassadeq on the accessory to murder charges, ruling that the evidence showed he knew the plotters planned to hijack and crash planes. It limited the number of counts, however, to the 246 people killed aboard the airplanes, and the 15-year sentence was restored.

The Associated Press contributed to this report.