Who is the victim?
Zelmkhan Khanushvili, a Georgian citizen, fought in the second Chechen war against Russian troops in the North Caucasus between 2001 and 2005.
He was a close ally of Chechen President Aslan Maskhadov, who masterminded the Chechen guerrilla war against Russia and was killed in a Russian special forces raid.
He fled to Germany after surviving an assassination attempt in the Georgian capital of Tbilisi in 2015. He applied for asylum in Germany but was not granted.
Why are suspicions about Russia's involvement?
A man was arrested as a suspect in the Khanghushvili assassination shortly after the killing, and the authorities alleged that he was trying to get rid of a bicycle, a pistol, and a wig, by throwing it into the nearby Spree River when he was arrested.
The German prosecutor says the suspect flew from Moscow to Charles de Gaulle airport in Paris, six days before the killing, under the pseudonym "Vadim S".
Vadim S first appeared on a Russian passport in 2015.
His full name is reportedly Vadim Sokolov, 49, but there are doubts about his true identity.
Who is the suspect?
According to the German prosecutor's statement, the defendant is Vadim K and his pseudonym Vadim S, but according to the Billings website for investigative journalism, the real identity of the detained man is Vadim Krasikov, born in August 1965 in Kazakhstan, then part of the Soviet Union.
The prosecutor says Vadim is also suspected in the murder of a Russian businessman in Moscow in 2013. The Russian businessman was killed in a similar manner to that of Khanjushevili, as a man approached him on a bicycle and shot his head from behind.
An arrest warrant against him was issued in Moscow in 2014 but was withdrawn in July 2015.