The FBI alert follows a Reuters report this week that said the agency had struggled to stop these hackers that are known to be skilled at using fake profiles and impersonations to trick a victim organisation's help desk into giving them access.
They were behind the September hacks into casino companies MGM Resorts International and Caesars Entertainment, but have intruded various organisations from telecom companies to healthcare groups, security researchers say.
The statement, issued jointly with the U.S. Cybersecurity and Infrastructure Security Agency, sheds new light into how these hackers operate.
Even after they've gained access into an organization's systems, the hackers keep checking its internal communication channels such as Slack, Microsoft Teams, and Microsoft Exchange online, for emails or conversations that might show if their breach had been discovered, the statement said.
The criminals "frequently join incident remediation and response calls and teleconferences, likely to identify how security teams are hunting them and proactively develop new avenues of intrusion in response to victim defenses," it added.
The FBI and CISA urged critical infrastructure organisations to implement a series of security measures they recommended and urged victim organisations to share information about the hacks with the agencies.
Everything from a sample ransom note, communications with the hackers, their cryptocurrency wallet information, or samples of malicious files could be useful, they said.
"FBI and CISA do not encourage paying ransom as payment does not guarantee victim files will be recovered," they said, adding that ransom payments may embolden the hackers into going after more targets.
(Reporting by Zeba Siddiqui in San Francisco; Editing by Nick Zieminski)
By Zeba Siddiqui