Chief Kathleen O'Toole said Thursday in a statement that both the officer and the adult woman were injured in the incident last Sunday. The officer arrested the woman in a domestic violence case.
Based on a review of in-car video, police officials decided additional investigation was warranted.
O'Toole did not identify the South Precinct officer. She says the officer is on paid leave while the investigation proceeds.
The chief says the department has also briefed the U.S. attorney and the FBI and asked that Washington State Patrol investigators assist Seattle police.
The City Council earlier this week approved the hiring of O'Toole, a former leader of Boston's police force. The Seattle department has been struggling to carry out a reform agreement under federal oversight.