That’s a real killer smile.

A Moroccan man convicted of helping the 9/11 terrorists carry out their plot flashed a huge grin as he flew home a free man on Monday after being released early from his meager sentence in Germany.

Mounir el Motassadeq was sentenced to just 15 years behind bars for accessory to murder in 2006, but received credit for time served after his initial arrest in 2001 — and was freed shortly before finishing even that stretch to be deported to his homeland.

German courts ruled that he was part of the so-called “Hamburg cell” with ringleader Mohamed Atta and two of the other extremists, and was aware the three planned to hijack and crash planes — if not all the details of the plot, which took 2,997 lives on Sept. 11, 2001.

El Motassadeq, who admitted to training at an al Qaeda camp in Afghanistan, helped “watch the attackers’ backs and conceal them” by paying their tuition and rent so they could keep up appearances as students in Germany as they plotted, the court found.

He maintained that he knew nothing of his friends’ plans to attack the US.

“I swear by God that I did know the attackers were in America,” he shouted in German at a sentencing hearing.

“I swear by God that I did not know what they wanted to do.”

Victims’ loved ones were horrified when news that el Motassadeq would soon be free first broke in August.

“He was found guilty of 246 counts of accessory to murder — one for each of the passengers who died on all the four hijacked flights that day,” Debra Burlingame, whose brother Charles was the pilot of the American Airlines plane that was flown into the Pentagon, told the Boston Herald at the time.

“It’s shocking he only got 15 years and this sends the message the cost of human life is cheap in Germany.”

El Motassadeq is one of only two men convicted for Sept. 11.

He was first convicted in 2003 of membership in a terrorist organization and thousands of counts of accessory to murder — accounting for all of those killed on the ground — but a federal court overturned the verdict in 2004 due to lack of evidence and sent the case back to Hamburg.

After a 2005 retrial, he was again convicted of membership in a terrorist organization, but was acquitted of accessory to murder because there wasn’t enough evidence that he knew of the hijackers’ plot.

He was sentenced to seven years at the time, but was freed in 2006 for an appeal.

Finally, later that year, the federal court reversed the acquittal of el Motassadeq on the accessory to murder charges — ruling that evidence showed he knew his pals were planning to hijack planes, but limiting the number of counts to the 246 people killed aboard the jets.

With Post wires