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.
But 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.
“We have the guy, but he isn’t talking and Russia isn’t helping — so we are stuck with this mystery,” said Patrick Sensburg, a conservative lawmaker who sits on the parliamentary committee that oversees the intelligence services.
“Russia should have an interest in helping if they are not involved,” Mr. Sensburg added. “The fact that they are stonewalling us inevitably looks suspicious.”
For Germany, the case has rekindled fears of Russian assassins roving freely around Europe, just over a year after Moscow was blamed for the poisoning in Britain of a former Russian spy, Sergei V. Skripal. The geopolitical stakes also are high, given Germany’s tumultuous but important diplomatic and economic relationship with Russia.
The Kremlin has denied any connection to the Berlin suspect, but it has also ignored German investigators’ requests for help. The victim, Zelimkhan Khangoshvili, fought against Russian troops in at least two wars in the early 2000s and had survived other assassination attempts before settling in Germany in 2016.
The German authorities have proceeded cautiously. The Berlin police are still in charge of the case. Putting it in the hands of federal investigators would signal official suspicions that the Russian state might be involved in the killing. Law enforcement officials briefed some German lawmakers about the suspect’s possible identity two weeks ago, but have yet to make that name public.
When the suspect was arrested, he was carrying 3,000 euros in cash, two hotel cards and a newly issued non-biometric Russian passport that identified him as Vadim Andreevich Sokolov. Investigators believe the name is fake.
The anonymous email, sent via the encrypted service ProtonMail, was short and written in poor English, European and American officials said. At least one foreign intelligence agency rated the tip as credible, and several are aware of it.
The email predicted that the suspect would not talk and described him as a Russian contract killer who had done time in a penal colony identified as IK-11. The prison, located in the town of Bor, 270 miles east of Moscow, is said to be reserved for former law enforcement and intelligence officers convicted of grave crimes, according to news reports.
Officials say the email also linked to a news article from the Russian-language aggregator Obzorcity about a 2005 trial in Moscow in which Vladimir Alekseevich Stepanov was sentenced to 24 years in prison. He was not scheduled to be released for another decade. The email’s implication was that he had been released early in order to carry out the mission in Berlin.
Independently, The New York Times reviewed court and government records in Russia and Russian-language news outlets and corroborated the main elements of the email tip. Vladimir Alekseevich Stepanov is a former major in the St. Petersburg police department.
In court records, Russian prosecutors said that Mr. Stepanov had used his position as a senior police official to provide cover and logistical support for a criminal gang that carried out a number of contract killings around Russia.
Mr. Stepanov was convicted of direct involvement in only one murder. In October 2003, he and an associate went to the home of Yelena Neshcheret, the owner of a St. Petersburg company that produced power tools. They waited in the courtyard and stabbed her to death. According to Russian investigators, Mr. Stepanov used a large trophy knife given to him as an award for his service during one of the wars in Chechnya.
German investigators interviewed by The Times said they could not be certain that the man they have in custody is the same as the St. Petersburg cop who moonlighted as a hit man. They have submitted a request to Russian officials to verify whether Mr. Stepanov was still in the specified prison and whether his file matched that of the Berlin suspect. So far, it is unanswered.
Times reporters also ran searches through millions of images on numerous photo databases and located two potential photographs of Mr. Stepanov from his trial in Moscow, one with his face partially obscured by the bars of a steel cage in the courtroom, the other in profile.
The Times provided a copy of the photographs to a European police officer who specializes in facial recognition. Asked to compare those photographs with recent pictures of the suspect in Berlin, the officer told The Times that he was 90 percent certain it was the same man.
But German investigators remain uncertain. Attempts to reach the sender of the mysterious email have been futile.
Investigators are wary of being lured into a trap, possibly as part of an effort by Russia or others to muddy the waters with disinformation.
It is not uncommon for Russian intelligence services to recruit prisoners and former criminals, experts say. Earlier this year, Oleg Smorodinov, a former police officer turned professional crook, told The New York Times of being recruited by Russian intelligence operatives and sent into Ukraine to kill a retired special forces commander.
A 2006 assassination attempt on Mr. Khangoshvili appears to fit that pattern and could shed some light on his killing in Berlin 13 years later.
That 2006 case also involved a former inmate who had been released early and ordered to kill Mr. Khangoshvili in the Republic of Georgia, where he was living at the time, according to Zurab Maisuradze, the former chief of Georgia’s antiterrorism center. Before he could do so, the Georgian authorities intercepted him, and arranged for him to record an encounter with his Russian handler, who was believed to be an officer with Russia’s domestic intelligence service, the F.S.B.
On the video recording, Mr. Maisuradze said, the handler is heard ordering the assassin to murder Mr. Khangoshvili. He hands over $20,000 in cash and a Makarov pistol with a silencer.
Experts say that outsourcing assassinations helps Russian government avoid the sort of geopolitical blowback that followed the poisoning of Mr. Skripal last year. Shortly after the British authorities determined that Moscow was most likely responsible for the attack, two dozen countries, including the United States, expelled more than 100 Russian diplomats in retaliation. The British authorities have indicted, in absentia, two decorated officers of the Russian military intelligence service known as the G.R.U. over the case.
“A professional kills with someone else’s hands,” said Oleksiy Arestovych, a retired officer in Ukraine’s military intelligence service. “A staff officer would never take part in a liquidation, except for the case of the G.R.U. in Salisbury, though that was total idiocy.”
In the Berlin killing, the suspect flew from Moscow to Paris on Aug. 17, traveling under the alias Vadim Sokolov and an expedited French visa that gave him freedom of travel in much of the European Union. He spent one or two nights in Paris, officials said, and then flew to Warsaw and checked into a hotel for five nights.
But after only three nights, on the eve of the murder, he departed for Berlin, leaving behind his luggage with clothes and a cellphone. Investigators believe he intended to return to Warsaw within 48 hours and travel back to Russia from there.
Officials say the killer almost certainly received logistical support — including the bike he was riding when he shot Mr. Khangoshvili; the electric scooter that was strategically parked to allow for a quick getaway; and the murder weapon. He was also probably briefed by accomplices on Mr. Khangoshvili’s movements.
Shortly before noon on Aug. 23, the killer tracked Mr. Khangoshvili to Kleiner Tiergarten, a public park in central Berlin. Mr. Khangoshvili was on his way to the mosque for Friday Prayer when the gunman approached him from behind on a bicycle and fired three shots with a silenced Glock 26 pistol, hitting him twice in the head and once in the torso.
The killer then hopped on his bike and rode away.