According to the Justice Department, the Peraire-Buenos exploited a flaw in popular MEV software called "MEV-boost", which is used by most Ethereum validators. By creating their own validators and "bait transactions", they were able to trick MEV bots into proposing transactions involving illiquid cryptocurrencies, which the brothers then frontran. They were able to create false signatures that tricked a MEV-boost relay into releasing information about upcoming blocks that they were able to tamper with.
The brothers were charged with conspiracy to commit wire fraud, wire fraud, and conspiracy to commit money laundering, and face up to 20 years in prison for each charge.
The Justice Department is describing the case as a "first-of-its-kind manipulation of the Ethereum blockchain". The case is an interesting one, as some believe the practice of MEV itself exploits Ethereum users. Others believe anything you can do with code should be allowed — "code is law". However, by signing false transactions and tricking the relay into releasing private information, the brothers' actions do seem to go beyond simply making profits in a "code is law" Wild West, and into the realm of actual fraud.