Officials have evacuated an airport in Flint, Michigan, where a witness says he saw an officer bleeding from his neck and a knife nearby on the ground.
On Twitter, Michigan State Police say the officer is in critical condition and the FBI was leading the investigation into Wednesday's incident.
Bishop International Airport posted on Facebook that passengers were safe and were being told to check for flight delays or cancellations.
Ken Brown told The Flint Journal he was dropping off his daughter at the airport and saw the officer bleeding from his neck. He says he saw a man detained by police and a knife on the ground.
"The cop was on his hands and knees bleeding from his neck," Brown said. "I said they need to get him a towel."
Cherie Carpenter, who was awaiting a flight to Texas told Flint TV station WJRT she saw the attacker being led away in handcuffs. She described the man in custody as appearing "blank, just totally blank".
Flint is about 80km northwest of Detroit.