Officers received a call at around 5.50pm on Saturday in Woodbridge, just outside Alexandria, Virginia, about a “verbal argument”, said Sergeant Jonathan Perok of Prince William County police.
He said a military serviceman was in custody and facing a capital murder charge.
Newly sworn in Officer Ashley Guindon died “as a result of her injuries sustained during the incident”, according to the police department.
Ms Guindon was a county police officer a few years ago and had left, then returned to the force, Corey Stewart, chairman of the Prince William County Board of Supervisors, said.
The department tweeted a picture of Ms Guindon on Friday with a message: “Welcome Officers Steven Kendall & Ashley Guindon who were sworn in. Be Safe!”