A March 14, 2014, file photo shows a warning sign outside facility of the TransCanada Keystone Pipeline in Steele City, Nebraska. (Reuters/Lane Hickenbottom)
A March 14, 2014, file photo shows a warning sign outside facility of the TransCanada Keystone Pipeline in Steele City, Nebraska. (Reuters/Lane Hickenbottom)

U.S. documentary filmmakers face prison for filming protest