U.S Park Police said yesterday they are investigating a brutality complaint filed by a Silver Spring woman who was stopped for a traffic violation just off the Baltimore-Washington Parkway early Sunday morning.

Police refused to discuss details of the complaint, but the motorist, Ellen Macdonald, 31, said the officer fractured her arm as she sat in her car protesting the traffic ticket. Macdonald said she did not get out of the car and was seated with her arms placed over the steering wheel, as the officer had ordered, when she was struck in the left arm.

Macdonald was treated for a fractured arm at Doctors' Hospital of Prince George's County where she was taken after the incident, according to a hospital spokesman.

Police said Macdonald had been charged with disorderly conduct after the incident occurred. They identified the officer named in Macdonald's complaint as Maria A. Gonzalez and said she would not be permitted to comment while the complaint is under scrutiny by the internal investigations division.

Macdonald said yesterday the incident occurred after she was stopped about 1 a.m. Sunday on an entrance ramp to the Goddard Space Flight Center just off the parkway. Macdonald said she told Gonzalez she had used the ramp, which is off limits to the public, because she was lost. When Gonzalez wrote her a ticket, Macdonald said she started to cry and protest.

Macdonald said she yelled at Gonzalez, "Why don't you just arrest me," and was then told to place her arms across the steering wheel and face forward. "The next thing I knew I was doubled over the front wheel. My arm was in pain in one part, numb in another."

Macdonald was taken by a rescue squad to the hospital where she was treated.

Macdonald said a few minutes before she was stopped by Gonzalez she had received a speeding ticket from another Park Police officer. The ticket from Gonzalez was for disobeying a sign, Macdonald said.