Cameras roll as authorities try to take control of the woman who first tried to breach White House security before heading to the Capitol.

A woman who tried to force her car through a White House security fence Thursday afternoon was shot and killed by police after a 12-block chase past the Capitol, which was locked down for a half-hour, authorities said.

The suspect — a dental hygienist with a history of mental issues, according to sources — had an 18-month old daughter with her who was not hurt, police said.

One Secret Service officer was struck by the woman's car, and a Capitol Police officer was injured when he slammed into a barricade during the pursuit. The unidentified police officer was treated and released Thursday night from MedStar Washington Hospital Center.

Slideshow: Shots fired on Capitol Hill / The U.S. Capitol was placed on lockdown following a car chase and shooting involving a woman who attempted to ram the White House gates. Launch slideshow

All the shots fired came from the officers involved in the pursuit, and the woman — identified as Miriam Carey, 34, of Stamford, Conn. — did not have a gun, law-enforcement sources said.

"She was using the car as a weapon," one source said.

Dramatic video showed officers with guns drawn surrounding the woman's black Infiniti before she suddenly sped away. Several shots could be heard as cops took off after her.

“This appears to be an isolated singular matter with no nexus to terrorism,” Capitol Police Chief Kim Dine said at an evening news conference as police and FBI agents converged on the woman's condo complex in Stamford.

President Barack Obama was briefed about the harrowing incident, which came in the midst of the government shutdown that has created a tense atmosphere on Capitol Hill.

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It started at 2:18 p.m. when Carey tried and failed to breach a White House checkpoint, a temporary "outer perimeter" fence at 15th St. and E St. NW, officials said.

As she fled east on Pennsylvania Ave., she struck a Secret Service officer, and a chase ensued.

Witnesses to the Capitol Hill shooting describe what they saw and heard.

Capitol Police caught up to her at Garfield Circle near the Capitol reflecting pool, but she sped off again. An officer in hot pursuit struck a barricade and was hurt, officials said.

The 12-block chase ended at Constitution Ave. and Second St. with Carey mortally wounded, police said. The child was removed from the car by a police officer and taken to the hospital.

The woman's motive was unknown, but investigators said they believe the woman may have been mentally unstable.

"They think she has mental health issues," said Rep. Mike McCaul, chairman of the House Homeland Security Committee, who was briefed by the FBI and Secret Service.

A senior law enforcement official with direct knowledge of the case told NBC News that relatives interviewed by law enforcement officers tell them she suffered from a fall a couple of years ago and began to suffer mental issues.

The Stamford apartment complex where Carey is believed to have lived was surrounded by police lines, fire crews and a decontamination area with tarp Thursday evening. Teams in Hazmat suits appeared to be entering the building just after 9 p.m.

Washington, D.C., Police Chief Cathy Lanier dismissed any suggestion that she had tried to breach security by accident and said the officers "acted heroically."

Travis Gilbert, who watched the high-speed chase from the roof of the Newseum, said there were several close calls as police cars raced after the Infiniti.

Authorities say no shots were fired at the White House during an attempted security breach. All the gunfire came at other locations and resulted in the driver's death.

"It was very dangerous," he said.

Frank Schwing, 57, a furloughed Commerce Department worker, said he was on the House side of the capitol when he saw police surround the car only to have the driver suddenly hit the gas.

"The sedan backed up and smashed into one of the cruisers, took off again around the south side of the Capitol," Schwing said. "And that's when I heard the gun shots. "

Rep. Gerry Connolly, D-Virginia, who was on the balcony talking to his colleagues, described a "burst" of gunfire from the House side of the Capitol, towards the House office buildings.

"It was like the first volley in a 21-gun salute," Rep. Matthew Cartwright, D-Penn., told MSNBC.

The FBI responded to the scene, and a helicopter landed in front of the Capitol to medevac the injured officer.

A message from the Capitol Police ordered anyone in a House office to "shelter in place." The House recessed, and the Senate went into a quorum call — dispensing momentarily with its official business — shortly thereafter.

“We’ve locked the doors. We closed the window shades. And we are awaiting further instructions,” Rep. Jared Polis, D-Colo., told MSNBC during the lockdown. “We’re more or less cut off here. We’re watching TV and just trying to figure out what happened.”

The lockdown was over around 3 p.m., but nerves were still jangled.

"Shaken is a good word to describe how I'm feeling," said Peter Plocki, a government worker furloughed during the shutdown who was on Capitol Hill to take a tour of the Supreme Court building and heard the shots.

Gun shots in front of Capitol. Cops scrambling. Not a joke — Luke Russert (@LukeRussert) October 3, 2013

The House reconvened at 3:30 p.m., and Rep. John Culberson, R-Texas, asked for a brief moment of silence in tribute to members of the Capitol Police injured in the incident. The House immediately pivoted back to debate over a small stopgap bill to reinstate funding for veterans’ affairs.

Senate Majority Leader Harry Reid, a former U.S. Capitol Police officer, called the injured officer at the hospital. "The only thing I do every day is to make sure you and everyone who works up here is safe," the officer said, according to Reid.

Congress has been locked for the past week and a half in a contentious debate over funding the government, a disagreement in which contributed to a government shutdown that began Monday.

Individuals can be seen running Thursday at the U.S. Capitol following gun shots.

Last night, Rep. Sean Duffy, R-Wisconsin, was the victim of a "minor incident" outside of the Capitol complex.

"A random individual, unknown to the Congressman, began screaming at him and grabbed his arm," a spokesperson for Duffy said in describing the incident. "Mr. Duffy was unharmed. He reported the incident in compliance with House security procedures. Congressman Duffy has requested no further action be taken and there will be no further comment on the matter at this time."

On September 16, a deadly shooting occurred at the Navy Yard just blocks south of the U.S. Capitol complex which contributed to a partial lockdown of the Capitol at that time.

A shooting on July 24, 1998 left two Capitol Police officers dead. And at a constituent event in her district in January 2011, then-Rep. Gabrielle Giffords, D-Ariz., was seriously injured and six others were killed in a shooting.

NBC News' Jonathan Dienst, Robert Windrem, Shimon Prokupecz, Michael Isikoff, Matthew Cole and Andrew Rafferty contributed to this report.

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