WASHINGTON (AP) — U.S. authorities are expressing increased alarm about an intrusion into computer systems around the globe that officials suspect was carried by Russia.

The cybersecurity unit of the Department of Homeland Security said Thursday that the hack “poses a grave risk to the Federal Government and state, local, tribal, and territorial governments as well as critical infrastructure entities and other private sector organizations.”

It was the most detailed comments yet from the Cybersecurity and Infrastructure Agency since reports emerged this weekend that government agencies including the Treasury and Commerce departments were among those whose secure data and email were penetrated by the sophisticated hack.

CISA also warned that it will be difficult to remove the malware inserted through network software. “The agency said that removing this threat actor from compromised environments will be highly complex and challenging for organizations.