IN APRIL 2019, a Russian man drove to a training base outside Moscow run by the Federal Security Service, or FSB, a successor to the Soviet KGB. The base, near the village of Averkiyevo, is equipped with multiple shooting ranges and is designed to train FSB officers in shooting techniques. The nonprofit journalism outfit Bellingcat has identified the man as Vadim Krasikov, and says that, based on cellphone data, he spent four days at the facility, an important clue about who was behind a murder in Germany last August.

Bellingcat says Mr. Krasikov, who traveled under a fake name, is the man charged by German federal prosecutors with the shooting in Berlin on Aug. 23 of Zelimkhan Khangoshvili, a former Chechen rebel field commander, who had fought Russia in the Second Chechen War. An ethnic Chechen and citizen of Georgia, Khangoshvili sought asylum in Germany after previous attempts on his life. He was shot in the head and shoulder from behind with a Glock-26 pistol. His alleged killer was apprehended after witnesses saw him ditch the weapon, a wig and a bicycle in the nearby River Spree.

At first, it wasn’t clear who was behind the assassination. Russia denied any responsibility. President Vladimir Putin’s spokesman, Dmitry Peskov, rejected any accusation of Russian involvement, describing it as “absolutely groundless.” Mr. Putin, in Paris on Dec. 9, said Khangoshvili was a “cruel and blood-thirsty” militant wanted by Russia. But as for who killed him, he added, “I don’t know what happened to him. It’s a criminal milieu, and anything can happen there.”

Now, however, Bellingcat and its partners, Der Spiegel and the Insider, have uncovered information that points directly to the FSB. According to the journalists, “This investigation conclusively establishes” that the agency “plotted, prepared, and perpetrated” the killing.

The investigation, based on cellphone metadata and cellphone tower connection records, documents calls and movements by Mr. Krasikov. He was frequently in contact with Eduard Bendersky, who heads an FSB veterans association and runs private security agencies that provide services to state-owned companies. The probers found that Mr. Bendersky and Mr. Krasikov spoke by phone at least 20 times in the period of February-August 2019, with the phone calls becoming more frequent in the month before the assassin’s trip to Berlin. The last call was Aug. 13, just before Mr. Krasikov departed Russia.

The journalists called Mr. Bendersky on the same phone — and he denied ever hearing of Mr. Krasikov. The data show Mr. Krasikov visited another FSB special operations center in Balashikha outside Moscow eight times, including just before he left Russia. He made stops in Paris and Warsaw before the shooting in Berlin’s Kleiner Tiergarten.

Germany has expelled two diplomats, saying Russia was not cooperating with the investigation, but that’s barely a slap on the wrist. The investigation by Bellingcat and its partners depicts a state-sponsored assassination on German soil, not unlike the earlier attempt to kill Sergei Skripal and his daughter in Britain and, before that, the murder of Alexander Litvinenko with polonium-laced tea in London. Germany must show that such crimes will not be tolerated.

Read more: