There is growing evidence that the murder of a former Chechen insurgent in Berlin last month was carried out on behalf of the Russian state, according to an investigation.

Russian authorities had to have been involved in the complex procedure of assembling a false identity for the alleged killer, who has been in police custody for over a month, the team of investigative journalists from Der Spiegel, Bellingcat, the Insider and the London-based Dossier Center, which traces Kremlin links to criminal activity, has claimed.

Norbert Röttgen, the chair of the Bundestag foreign affairs committee, told Spiegel: “The crime appears to have considerable and clear political fingerprints.” He said that for reasons he could not comprehend the case had not been passed on to the federal public prosecutor’s office.

Zelimkhan Khangoshvili, 40, was shot dead at close range in the Kleiner Tiergarten park while he was on his way to the mosque on 23 August.

Facebook Twitter Pinterest The spot where Khangoshvili was killed. Photograph: Sean Gallup/Getty Images

The suspected killer, named by German police as Vadim Sokolov, which is not thought to be his real name, tried to flee by bicycle but was apprehended by police after passersby had reported seeing him throwing a wig and gun into a river.

Police investigators were quick to compare the killing of Khangoshvili to the attempted murder of Sergei Skripal and his daughter in Salisbury by suspected Russian agents last year.

The suspect has so far refused to answer police questions. The Berlin prosecutor’s office has described him as “behaving like a professional”.

The journalists’ investigation has found there is no one registered in the Russian passport records office as Vadim Sokolov. In addition, the address in his Schengen area visa application is false. His passport was found to have been issued by a department of the Russian interior ministry, which has in the past been responsible for issuing passports for agents of the Russian military secret service, the GRU, the journalists say.

They claim they have also found evidence in a Russian database of attempts to create a credible false identity under the name Sokolov, shortly before the murder. The procedure recorded there is similar to that found during the Bellingcat exposure of the identity of the alleged Skripal attacker. Sokolov was also registered in the Russian tax database in June 2016, when he would have been 49, despite the fact that Russian citizens are registered as taxpayers as soon as they start work, the journalists found.

Only days after receiving a tax number and a Russian passport, he applied for a Schengen area visa at the French embassy, according to the investigation. Sokolov was not to be found at the address with which he registered with the tax office.

On Thursday, the New York Times reported claims that Sokolov may in fact be Vadim Andreevich Stepanov, a convicted murderer.

But the team of investigative journalists, with the help of forensic experts at Bradford University, have found no match between photographs of the suspect and Stepanov. There are also reports that Stepanov is in prison.

Khangoshvili fought in the second Chechen war against Russia. He later worked for several years as an informant and negotiator for the Georgian and Ukrainian counter-terrorist authorities. Following numerous threats and an assassination attempt on him, he moved to Ukraine and then fled to Germany where he sought asylum.

The Kremlin has denied any involvement in his killing.