Hackers who paid $40 for a sinister software program called Blackshades stole sexually explicit photographs from personal computers, trained victims' webcams on them using a remote access tool, and even sent threats to victims who tried to block the online intrusions, prosecutors said Monday as they announced charges against users of Blackshades.

Justice Department officials said Blackshades, which had thousands of customers and infected about half a million computers worldwide, underscored the global threat of cybercrime.