The Stamford police roped off and evacuated Ms. Carey’s condominium building Thursday afternoon, and bomb units stood by. Neighbors in Connecticut said Ms. Carey had a child, but it was unclear if the toddler in the car was hers.

Image Miriam Carey Credit... Advanced Periodontics

The chase began at 2:12 p.m. when Ms. Carey, who was driving a black two-door Infiniti with Connecticut plates, tried to ram through a White House checkpoint at 15th and E Streets Northwest. “The guys ran to try to stop her, and she wasn’t going to slow down, so they jumped aside,” said B. J. Campbell, a tourist from Portland, Ore., who was standing near the White House. “One of the guys grabbed one of those little metal fence sections and shoved it in front of her, across the driveway. She hit the brakes slightly and tried to get around it on the right, but the guy shoved it in front of her again, to try to keep her in.”

Mr. Campbell said the woman “hit the gas, ran over the barricade” and hit the officer, who flipped onto the hood of the car and “rolled off into the gutter.”

“After she ran him down, she gunned it, and she just went screaming down Pennsylvania Avenue,” he said. “They were busy calling on their phones, on the radios. It was like poking a hornet nest. There were guys everywhere. I didn’t see anyone with their guns out, but they were sure busy.”