German prosecutors are considering whether to open an investigation into the alleged leader of a Rwandan rebel group, who was arrested last week.
Mr Murwanashyaka's assets were frozen by the UN Security Council
Ignace Murwanashyaka is suspected of involvement in war crimes in Rwanda or the Democratic Republic of Congo.
Meanwhile, Mr Murwanashyaka is appealing against moves to deport after his refugee status was withdrawn.
Rwanda has requested his extradition. Mr Murwanashyaka is expected to appear in a German court on Tuesday.
Mr Murwanashyaka is believed to be the commander of Democratic Liberation Forces of Rwanda, a group operating in eastern DR Congo - some of whose members are suspected of involvement in the 1994 Rwandan genocide.
Thousands of Rwandan Hutu militiamen entered eastern DR Congo after the genocide in which about 800,000 people - mostly ethnic Tutsis - were killed.
The rebels' presence has led to years of fighting in eastern Congo and Rwanda has twice invaded, saying it is trying to wipe them out.
Reports say Mr Murwanashyaka has lived in Germany for several years but his residence permit was withdrawn as a result of United Nations Security Council sanctions.
He was among 15 people whose assets were frozen by the Security Council last November for suspicion of involvement in war crimes in Rwanda or DR Congo.
Mr Murwanashyaka was arrested on Friday suspected of violating immigration laws, and was ordered to be detained for three months pending possible deportation.
He has filed an appeal against the detention order, a court spokesman said.