I suppose they seized the computers of Iranians living, or traveling in the US. Or could there be another way? Did the US government seized BTC at an American exchange, where some customers happen to be Iranian citizens?
The articles on this subject are very vague and don't mention the exact details: https://bitcoinist.com/us-government-confiscates-iranian-bitcoin-holder-assets/
Like gentlemand mentioned, I think it was Iranian account holders at American exchanges. I've heard complaints from Iranians going back to last year about Bittrex locking their accounts and providing zero recourse.
It really sucks, because it was the fault of the exchanges. They shouldn't have been doing business with residents of sanctioned countries, but they did, and their terms allowed for it. Then overnight, presumably by some intervention by US authorities, their accounts began being locked.
Could u plz explain how it is possible through an exchange?
Exchanges are just centralized businesses. They control the private keys to the cryptocurrency in your "account." They handed the coins over to the US government.