WASHINGTON – Democratic Sen. Al Franken wants FBI Director James Comey to explain why federal anti-terrorism law enforcement officers are investigating Dakota Access pipeline protesters.

Recent reports said agents from the FBI's Joint Terrorism Task Force tried to question at least three people who were protesting the construction of the pipeline through North Dakota. The federal officers visited the protesters' residence, without a warrant or a subpoena, an action that Franken said he worries could chill free speech.

The protesters declined to cooperate with the agents, Franken's office said.

"Make no mistake, where political demonstrations threaten public safety and security, police and federal law enforcement agencies have a duty to protect people," Franken wrote in a letter to Comey last week. "But … I fear that the federal government's posture conflates constitutionally protected conduct with actions that threaten our national security."

Hundreds of protesters decamped from the Standing Rock Sioux reservation after Bureau of Indian Affairs agents issued a Feb. 27 deadline for them to leave. The demonstrators, many tied to Indian tribes, are protesting the construction of a 1,100-mile oil pipeline they say could harm the reservation's drinking water.

President Donald Trump, reversing an Obama administration decision, is allowing Dallas-based Energy Transfer Partners to finish building the pipeline.

Franken asked Comey to get back to him by April 20. The FBI had no immediate response.