Russian cybersecurity experts have scaled back cooperation with Western contacts after one of their number was arrested in Moscow on treason charges, making it harder to fight global online crime, U.S. law-enforcement and industry sources say.

Despite acrimonious relations between Russia and the United States in recent years, experts on cybersecurity in both countries say their law enforcement agencies and private firms had been working together more closely behind the scenes to fight financial fraud and other crimes committed online.

But at least some of that cooperation appears to have come to a sudden halt since Ruslan Stoyanov, head of the computer incidents investigation team at Russian cybersecurity firm Kaspersky Lab, was arrested in December on suspicion of treason.