Hans Staiger was a German national and an Al-Qaeda agent known as "The Recruiter".
In addition, Staiger was tasked with spreading the beliefs of the terrorist group across Europe and getting people to join the group, thus increasing their numbers.
The European authorities never managed to track him down however, and no photos/DNA of him were ever obtained, but only some recorded calls.
At an unknown time, Staiger was relocated to the US where he was tasked with recruiting Muslim US marines into the terrorist group, and also with assassinating all of those who turned down the offer, including Abdul Bakr.
Using a Muslim mosque as a recruitment center, his activities were investigated by the FBI when the mosque owner Khalid Mohammed Bakr (also father of the deceased marine) asked them to monitor the mosque to detect any terrorist activity.
NCIS eventually began investigating as well when they find Abdul's corpse, and after they found out they were using a special poison that was almost undetectable to kill the marines to make it seem like the marines died of natural causes.
The team eventually found out evidence against Captain James Mills, the recruiter of Abdul and other Muslim marines. The marine cut a deal to get civilian trial and NOT be prosecuted as a terrorist and in exchange gave out the location of Staiger, who lived in a room at a hotel.
When they reached the hotel room, however, they found Staiger's dead body lying on the bed, holding the dead Marine's prayer beads, and realize Khalid had already killed the German terrorist to get revenge for the death of his son.
Gibbs and Ziva approached the body and eventually decide that justice had been served, and that no further investigation was necessary, thus closing the case for good.