Russian assassins have also been busy.

In October, The New York Times revealed that a specialized group of Russian intelligence operatives — Unit 29155 — had for years been assigned to carry out killings and political disruption campaigns in Europe. Intelligence and security officials say the unit is responsible for the assassination attempt last year against Sergei V. Skripal, a Russian former spy in Britain; a failed operation in 2016 to provoke a military coup in Montenegro; and a campaign to destabilize Moldova.

Western intelligence agencies now know the name of the unit’s commanding officer, Maj. Gen. Andrei V. Averyanov, and the location of its headquarters in Moscow. Based on interviews with officials in Europe and the United States, it is also now clear that the assassination attempts against Mr. Gebrev served as a kind of Rosetta Stone that helped Western intelligence agencies to discover Unit 29155 — and to decipher the kind of threat it presented.

Image Maj. Gen. Andrei V. Averyanov Credit... Agence France-Presse — Getty Images

Since the original Times story, more information has come to light, including the true identities of some of the unit’s members and other possible activities in Spain and France. This month, Germany expelled two Russian diplomats as punishment for the daylight assassination in Berlin of a former Chechen rebel commander, though it is unclear whether operatives from 29155 were involved.

Security and intelligence officials are still working to understand how and why the unit is assigned certain targets. Even now, investigators have not determined the precise motive in the Gebrev case. Most likely, intelligence officials say, Mr. Gebrev was a target because of the way his business rankled the Kremlin: his arms sales to Ukraine, his company’s intrusion into markets long dominated by Russia, and his efforts to purchase a weapons factory coveted by a Russian oligarch.

Mr. Gebrev says he also believes that local business rivals or politicians might somehow be involved.

“I have been thrown to the wolves,” Mr. Gebrev said in an interview. “But why and how, I’m still asking myself.”