Monaco said investigators had seized 63.7 Bitcoins, now valued at about $2.3 million, paid by Colonial after last month's hack of its systems that led to massive shortages at U.S. East Coast gas stations.
The Justice Department has "found and recaptured the majority" of the ransom paid by Colonial, Monaco said.
An affidavit filed on Monday said the FBI was in possession of a private key to unlock the hackers' Bitcoin wallet. It was unclear how the FBI gained access to this key.
A judge in San Francisco approved the seizure of funds from this "cryptocurrency address," which the filing said was located in the Northern District of California.
Colonial Pipeline had said it paid the hackers nearly $5 million to regain access. Bitcoin's value has dropped in recent weeks, trading at around $36,000 on Monday after hitting $63,000 in April.
"Today, we've turned the tables on DarkSide," said Monaco, referring to a ransomware group widely believed to have been behind the crippling fuel pipeline attack.
The hack caused a shutdown lasting several days, leading to a spike in gas prices, panic buying and localized fuel shortages. It posed a major political headache for President Joe Biden as the U.S. economy was starting to emerge from the COVID-19 pandemic.