She had a toddler with her, confounding everyone who watched the car crash through barriers and lead police through the heart of high-security Washington.

Law enforcement officials said the black Infiniti was registered to a 34-year-old mother named Miriam Carey, a dental hygienist from Stamford, Conn. They think it was Carey, with her 1-year-old daughter sitting behind her, who flattened barricades outside the White House, struck officers and then set off at high speed down Pennsylvania Avenue.

D.C. police confirmed that the driver was shot and killed after careening around the Capitol grounds and crashing at Second Street NE. There was no sign that she was armed, police said.

Video images showed a young child, her hair in braids, being carried by an officer to the back of a patrol car.

The initial portrait of Carey that emerged suggested a person unlikely to be found in the center of such violence. Carey, according to public documents, friends and family members, had finished college and established a work history as a dental hygienist.

Police had a confrontation with a woman in a black car and engaged in a brief car chase with her before shots were fired outside the U.S. Capitol on Thursday. (Thomas LeGro/The Washington Post)

Her sister, Amy Carey, a Brooklyn nurse, was incredulous when she was reached Thursday afternoon and told what had happened.

“That’s impossible. She works, she holds a job,” said Amy Carey, who confirmed that her Stamford-based sister drove a black car. “She wouldn’t be in D.C. She was just in Connecticut two days ago, I spoke to her. . . . I don’t know what’s happening. I can’t answer any more.”

About 11 p.m. Thursday, at least a dozen local police cars, fire trucks and ambulances lined a street in Stamford outside the apartment complex where Miriam Carey reportedly lived. Sections of the complex were cordoned off with yellow crime tape.

Eric Sanders, an attorney for the Carey family, said outside the Brooklyn home of another sister, Valarie Carey, that the family was struggling to absorb the news. He said he would release an official statement Friday.

“We have to mend the family first and find out what happened in D.C.,” Sanders said.

People who knew Carey described her as friendly and dedicated.

Angela Windley, 33, befriended Carey while both attended Hostos Community College in the Bronx and graduated with an associate’s degree in dental hygiene. Officials at Brooklyn College confirmed that a student named Miriam Carey graduated in 2007 with a bachelor’s degree in health and nutrition science.

Windley said she and Carey went on a vacation to Cancun together about three years ago but that she had lost touch with Carey in the past two years or so after Carey stopped replying to messages that Windley left on Facebook.

“She was really just a sweet and nurturing person,” said Sara Vega, a former schoolmate of Carey’s in Brooklyn, where Carey was born and raised. Vega, who now lives in Georgia, said she hadn’t seen Carey in recent years but that the two had been in touch occasionally on Facebook. “I know she had a daughter about a year ago.”

Windley said she and Carey had attended the same high school but did not become friends until their days at the community college. Carey had four sisters, including one who was a longtime New York police officer.

Carey “wanted to have a better life,” Windley said. “The neighborhood we both grew up in wasn’t the greatest, and she always talked about getting out.”

Carey moved to Stamford after “some problems with her landlord,” Windley said, “but the Brooklyn rents were getting so high . . . that’s how she landed in Stamford.”

Carey was “always very professional” and “very focused,” according to Windley. “She wanted better, always.”

Windley said she knew of no connections to the District that Carey might have had and had never heard Carey express any opinion about the government. “This is very shocking,” Windley said as she fought back tears during a telephone interview.

During her friendship with Carey, Windley said she had never witnessed Carey lashing out in anger “beyond, you know, normal girl stuff, like, ‘What is up with her?’ about another girl, but nothing crazy. Some sharp words, that would be it.”

Carey also had not shown signs of mental illness during the time Windley knew her and “was not one to even talk about politics.”

Carey did not have many friends, Windley said, and could seem “arrogant. If there was a negative, people said that was it. She could be sort of conceited, like she knew everything.”

That trait surfaced at school during their training and was off-putting to some of the faculty, Windley said. “But that was the most negative thing you could say about her.”

Carey also was a good driver with a good sense of direction, Windley said. “She wasn’t one to get all crossed up, she knew maps — I even recall saying that I hoped I became as good a driver as she was, since I can get lost in a box.”

In a news conference, D.C. Police Chief Cathy L. Lanier said the incident did not appear to be an accident, noting there was “a lengthy pursuit.”

Told about the confrontation on Capitol Hill, Windley said: “I would have pulled over, stopped. Why wouldn’t she? Why would she have done this? Why? And why with her daughter in the car? Why would she do something like that?”

Alice Crites, Jennifer Jenkins, Colum Lynch and Matt Zapotosky contributed to this report.