
Article:
https://www.smithsonianmag.com/smithsonian-institution/a-federal-agents-laptop-held-the-keys-to-seize-36-billion-in-stolen-bitcoin-heres-how-it-ended-up-at-the-smithsonian-180985485/Podcast version:
https://www.si.edu/sidedoor/bitcoin-bank-heistIn 2016 Bitfinex was hacked, and about 120k Bitcoins were stolen.
A hacker responsible for stealing 119,754 Bitcoin in a 2016 hack on the Bitfinex cryptocurrency exchange was sentenced to five years in prison by U.S. authorities.
The man, Ilya Lichtenstein, was arrested in February 2022 in Manhattan following a lengthy investigation led by the IRS, HSI, and the FBI, which managed to recover roughly 80% of the stolen cryptocurrency (94,000 Bitcoin)
At the time of the theft, the 119,754 bitcoins were worth $78,000,000 but equaled $3.6 billion at the time of the seizure.
They were able to seize the Bitcoins because they ended up having access to their computer, where he had the key to most of the stolen Bitcoin:
IRS-CI Special Agent Chris Janczewski, working closely with Department of Homeland Security investigators and prosecutors from the Department of Justice, traced the stolen Bitcoins. He treated the blockchain as a digital crime scene that eventually allowed him to identify the suspected perpetrators, search their home and seize their digital files onto his laptop. Within the tranche of seized files, Janczewski discovered the digital keys to the majority of the Bitcoins stolen from Bitfinex, enabling him and his law enforcement colleagues to seize funds worth approximately $3.6 billion, as valued at the time of the seizure in February 2022, and arrest Lichtenstein and Morgan. Today, those funds are valued at about $8.6 billion.
It is not clear from the article if the keys found were encrypted with a password or not, but eventually the Federal agent's laptop was used to access the Bitcoins with this seized key. It is this laptop that was acquired by the Smithsonian to be showcased in their exhibition called "
The Value of Money”:

You can have a look at the exhibition either in person, or online:
https://americanhistory.si.edu/explore/exhibitions/value-money/online