The group behind a global cyber espionage campaign discovered malicious computer code deployed with links to spy tools previously used by suspected Russian hackers last month, researchers said Monday.

Investigators from Moscow-based cybersecurity firm Kaspersky said the “backdoor” used to compromise up to 18,000 SolarWinds customers looked like malware linked to the “Turla” hacking group, which authorities said Estonian, operates on behalf of the Russian security service FSB. The results are the first publicly available evidence to support U.S. claims that Russia orchestrated the hack, which compromised a range of sensitive federal agencies.

Moscow has repeatedly denied these allegations.

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