The men who killed Maksim Kuzminov wanted to send a message. This was obvious to investigators in Spain even before they discovered who he was. Not only did the killers shoot him six times in a parking garage in southern Spain, they ran over his body with their car.

They also left an important clue to their identity, according to investigators: shell casings from 9-millimeter Makarov rounds, a standard ammunition of the former Communist bloc.

“It was a clear message,” said a senior official from Guardia Civil, the Spanish police force overseeing the investigation into the killing. “I will find you, I will kill you, I will run you over and humiliate you.”

Mr. Kuzminov defected from Russia to Ukraine last summer, flying his Mi-8 military helicopter into Ukrainian territory and handing the aircraft along with a cache of secret documents to Ukrainian intelligence operatives. In doing so, he committed the one offense President Vladimir V. Putin of Russia has said again and again he will never forgive: treachery.

His killing in the seaside resort town of Villajoyosa last month has raised fears that Russia’s European spy networks continue to operate and are targeting enemies of the Kremlin, despite concerted efforts to dismantle them after Mr. Putin invaded Ukraine in 2022.