US companies that employ illegal foreign workers have long operated in a kind of lawless nirvana: the government seldom pursued them for breaking the law, and neither did private plaintiffs.

Now, that may be about to change: last week the government signalled an aggressive crackdown against companies that employ illegal immigrants, arresting more than 1,000 people in the largest such action in US history. In addition, US employers face legal action on a second front: private lawyers are using America’s tough anti-mafia law to target companies that employ undocumented workers.