BERLIN — At the outset, the German officers investigating the brazen daylight assassination of a former Chechen separatist commander were lucky.

The killer, a man in a wig who had gunned down the victim in a Berlin park, was spotted by two teenagers, who called the police. Officers arrested the suspect near the Spree River as he stepped from a clump of shrubbery, with his wig gone, his clothes changed, his beard shaved off, and a Russian passport in his pocket.

Days after the Aug. 23 killing, investigators received an email from an anonymous sender. It suggested that the suspect was a hit man who had been released from prison by the Russian authorities in order to carry out the assassination in Berlin. It claimed his real name was Vladimir Alekseevich Stepanov, though authorities worried the email could be a hoax.

Since then, that early luck appears to have run out. Investigators have a suspect in custody, but they aren’t certain who he is. The man with a shaved head, a hangdog face and arm tattoos, has mostly stayed silent, meeting only consular officers from the Russian Embassy.