Best guess is that with the F1 races coming soon with what is expected to be the largest cashflow through Vegas ever, that MGM Resorts IT found issues in an audit in preparation for that massive event, found anomalies, and pulled the rip cord to shut everything down till they could sort out what systems were actually hit.
That is materially different than a massive hack effecting all these various systems though.
1. https://www.reviewjournal.com/business/casinos-gaming/mgm-re...
"MGM Resorts recently identified a cybersecurity issue affecting some of the Company’s systems. Promptly after detecting the issue, we quickly began an investigation with assistance from leading external cybersecurity experts,”
"We also notified law enforcement and took prompt action to protect our systems and data, including shutting down certain systems."
The systems are down due to MGM shutting them down, not the active attack shutting things down.
https://www.politico.com/news/2022/01/14/russia-colonial-pip...
...so they decide to hit casinos instead!
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That is quite a guess!
Outlook handles their email.