Federal investigators spent years hunting for clues in the 2016 hacking of the Bitfinex cryptocurrency exchange, when thieves stole bitcoin now worth $4.5 billion. In the end, what helped lead them to two suspects was something much more quotidian: a $500 Walmart gift card.

That card and more than a dozen others like it, including for Uber, Hotels.com and PlayStation, were linked to emails and cloud service providers belonging to a young Manhattan couple, Ilya “Dutch” Lichtenstein and Heather R. Morgan, according to a criminal complaint. Authorities arrested the couple after seizing $3.6 billion worth of bitcoin allegedly in their control—the Justice Department’s largest financial seizure ever.