Federal authorities expressed increased alarm Thursday about a long-undetected intrusion into U.S. and other computer systems around the globe that officials suspect was carried out by Russian hackers. The nation’s cybersecurity agency warned of a “grave” risk to government and private networks.

Hackers broke into the networks of federal agencies in attacks revealed just days after US officials warned that cyber actors linked to the Russian government were exploiting vulnerabilities to target sensitive data. The FBI and the Department of Homeland Security’s cybersecurity arm are investigating what experts and former officials said appeared to be a large-scale penetration of US government agencies. “This can turn into one of the most impactful espionage campaigns on record,” said cybersecurity expert Dmitri Alperovitch.