Caesars paid around $15 million in ransom to regain access to its systems from the hackers, according to reporting by the Wall Street Journal.
Some have been collecting evidence leading to the hackers' identities and are assisting law enforcement, according to the five insiders.
The sources say that, following the September casino hacks, the FBI's investigation took on new urgency.
Mandia didn't respond directly when asked whether Scattered Spider's identities were known to law enforcement.
But he did say that there was no excuse for not arresting hackers who operated from the West.
Persons:
Bridget Bennett, Michael Sentonas, Sentonas, Alphabet's, James Foster, Foster, cybercriminals, gona, Kevin Mandia, Mandiant, you've, Zeba Siddiqui, Raphael Satter, Christopher Bing, Chris Sanders, Claudia Parsons
Organizations:
MGM, MGM Resorts, REUTERS, FRANCISCO, U.S . Federal Bureau of Investigation, FBI, MGM Resorts International, Caesars Entertainment, Industry, Reuters, Department of Justice, Caesars, Wall Street, Palo Alto Networks, Microsoft, ex, Telegram, Thomson
Locations:
Las Vegas , Nevada, U.S, WASHINGTON, America, Palo, American, Baltimore , Maryland, United States, Newark , New Jersey, sextortion, San Francisco, Washington