In the old days, criminals liked their ransom payments in briefcases full of unmarked bills.

These days, there’s a new preferred method for hostage takers: the virtual currency Bitcoin.

In a modern day version of a mob shakedown, hackers around the world have seized files on millions of computers, taken down public websites and even, in a few cases, threatened physical harm. The victims — who have ranged from ordinary computer users to financial firms and police departments — are told that their only way out is through a Bitcoin payment that is sometimes more than $20,000.

One set of attackers, believed to be based in Russia and Ukraine, collected about $16.5 million in Bitcoins in a little over a month, primarily from victims in the United States, according to the security firm Sophos.

Criminals like the virtual currency because it can be held in a digital wallet that does not have to be registered with any government or financial authority — and because it can be easily exchanged for real money. At the moment, a single Bitcoin can be sold online or on the street for around $290.