An investigation by media outlet The Insider has identified a decorated officer with Russia's Federal Security Service (FSB) as a possible second suspect in the murder of a Georgian man in a central Berlin park last year.

A 40-year-old Chechen with Georgian citizenship was gunned down with three shots in Berlin's Tiergarten park on August 23, 2019. Police caught the suspected hitman the same day after he was seen throwing a wig, bicycle and a gun into the Spree River. He has been in custody ever since.

German prosecutors announced in June that they had charged a Russian man named as Vadim K., alias Vadim S. with the killing. They allege the assassination was likely ordered by Moscow — a claim Russia denies.

Read more: Berlin murder further strains Germany-Russia relations

In the report published on Saturday, investigative website The Insider said a 39-year-old member of Russia's FSB likely helped prepare the assassination. The Moscow-born suspect allegedly used a false identity to travel by car from Russia to Poland via Belarus, before possibly continuing on to Germany, around two weeks before the killing, according to the publication. He then returned to Russia on August 7, 2019.

The report said he had used a fake passport that was issued on the same day, and from the same batch, as that of the defendant Vadim K. Both of their visa applications said they were employed by the St. Petersburg company ZAO RUST, The Insider said.

The case deepened the diplomatic rift between Russia and Germany. In December, Berlin expelled two Russian diplomats after accusing Moscow of failing to cooperate in the investigation of the murder.

Earlier this month, German Foreign Minister Heiko Maas warned that Germany may consider taking further action against Russia, including sanctions, if prosecutors' allegations that Moscow was behind the assassination are ultimately proven in court.