The FBI is examining whether federal laws were violated by a Utah police detective seen on video assaulting and arresting a nurse after she refused to allow taking a blood sample from an unconscious patient.
The July 26 incident involving Salt Lake City police Detective Jeff Payne is already being investigated by state authorities, who requested the help of the FBI on Thursday.
Sandra Yi Barker, a spokeswoman at the FBI's Salt Lake City field office, said agents already opened a review of the matter after videos of Payne forcefully arresting University of Utah Hospital nurse Alex Wubbels emerged last week.
"We are there to support and assist them as needed, but we also have our own review going on at the same time," Barker said.
Ms Barker stressed that a "colour of law" review, which examines whether law enforcement officials exceeded their authority, was a preliminary step that may or may not lead to a formal investigation.
"Colour of law" violations include false arrest and the use of excessive force by police, according to the FBI's website.
Video from Salt Lake City police officers' body cameras showed Payne handcuffing Ms Wubbels and arresting her after she refused to draw blood from a comatose truck driver who had just been brought in, because Payne had neither a warrant nor the patient's consent.
The driver had been in a crash with a vehicle driven by someone fleeing police.
Salt Lake County District Attorney Sam Gill, whose office is investigating the incident, said he requested the FBI's help because "there continue to be issues that go beyond merely a criminal investigation."
Payne, who is currently on administrative leave from the police force, was fired on Tuesday from his second job as a part-time ambulance driver.
Gold Cross Ambulance service president Mike Moffitt told Reuters that Payne's termination followed comments he made on the video suggesting that he would bring transients to University of Utah Hospital, while transporting "good" patients to another facility.
"Those remarks are just not reflective of our company's philosophy and the service we provide, and because of that behaviour we felt we had to separate ways," Mr Moffitt said.