The FBI claimed it recovered millions of dollars in Bitcoin paid as ransom during the attack against the Colonial Pipeline — a feat that is now generating more questions than answers.

The Justice Department and the FBI announced Monday it had seized 63.7 bitcoins (worth about $2.3 million at the time) from a Bitcoin wallet thought to be controlled by cybercriminals tied to a Russia-based collective called DarkSide, which operates off a “ransom as service” model. Recovery of the cryptocurrency ransom from its presumably savvy holders, especially in such a short time, left...