The colectivos aren't nearly as big as Venezuela's armed forces - they number perhaps 5,000 to 7,500 members nationwide, most of them in cities, according to Alejandro Velasco, a history professor at New York University who has studied the phenomenon. But they help explain how Maduro has remained in power even as the country's economy and poorly maintained power grid have broken down. The paramilitary forces are nimble and committed - and they have an extraordinary ability to sow terror.