The family of a woman who was shot to death outside the U.S. Capitol after trying to ram her car through a White House barrier said Friday they want to know why she had to die.

Speaking at a press conference late Friday in Brooklyn, Miriam Carey's sister Amy Carey-Jones said Friday that her sister "seemed OK" the last time they spoke more than a week ago.

"We're still very confused as a family why she's not still alive," Amy Carey-Jones said late Friday, speaking of her 34-year-old sister. "I really feel like it's not justified, not justified."

Another sister, retired New York City police officer Valarie Carey, said there was "no need for a gun to be used when there was no gunfire coming from the vehicle."

Her family said she had been suffering from postpartum depression with psychosis but was not dangerous.

Carey-Jones said her sister had been on medication for postpartum depression but was being taken off the drugs under medical supervision.

"They told her she could get off medication," Carey-Jones said, adding, "There were no indications she was unstable."

But interviews with some of those who knew Miriam Carey, of Stamford, Conn., suggested she was coming apart well before she loaded her 1-year-old daughter into the car for the 275-mile drive to Washington on Thursday.

Carey had suffered a head injury in a fall and had been fired as a dental hygienist, her former employer said.

Rep. Michael McCaul, chairman of the House Homeland Security Committee, told Fox News that investigators believed Carey suffered "serious" mental issues and that President Obama was trying to communicate with her through radio waves.

Valarie Carey questioned the characterizations of her sister's deteriorating mental health and said her Miriam Carey "did not believe the president or any government official was going to do her harm."

After ramming the barricades at the White House, the apparently unarmed Carey led police on a chase down Constitution Avenue to the Capitol, where she was shot in a harrowing chain of events that led to a brief lockdown of Congress. Carey's daughter escaped serious injury and was taken into protective custody.

A lawyer for Carey's sisters, Eric Sanders, attributed the shooting to the "siege mentality" that has developed because of terrorism.

"We're afraid of everything. We're afraid of ourselves now," Sanders said, adding that the family would do its own investigation, beginning with an autopsy.

The House and Senate both abruptly suspended business, a lawmaker's speech cut off in mid-sentence, as the Capitol Police broadcast a message over its emergency radio system telling people to stay in place and move away from the windows.

The woman's car at one point had been surrounded by police cars and she managed to escape, careening around a traffic circle and past the north side of the Capitol. Video shot by a TV cameraman showed police pointing firearms at her car before she rammed a Secret Service vehicle and continued driving. Metropolitan Police Chief Cathy Lanier said police shot and killed her a block northeast of the historic building.

Carey's neighbors in Stamford were shocked to learn the driver's identity and see her gleaming black Infiniti wrecked outside the Capitol in TV footage.

Erin Jackson, her next-door neighbor on the building's ground floor, said Carey doted on her daughter, Erica, often taking her on picnics.

"She was pleasant," Jackson said. "She seemed very happy with her daughter, very proud of her daughter."

Carey's mother, Idella Carey, told ABC News Thursday night that her daughter began suffering from post-partum depression after giving birth to her daughter, Erica, last August.

Idella Carey said her daughter had "no history of violence" and she didn't know why she was in Washington, D.C., on Thursday. She said she thought Carey was taking Erica to a doctor's appointment in Connecticut. Authorities also discovered an unidentified white powder at her Connecticut home.

Dr. Brian Evans, a periodontist in Hamden, an hour's drive northeast of Stamford, said Carey was fired from her job at his office about a year ago but wouldn't say why. He said Carey had been away from the job for a period after falling down a staircase and suffering a head injury and it was a few weeks after she returned to work that she was fired.

On Sept. 16, a man killed 12 people in a shooting rampage at the Washington Navy Yard before dying in a gunbattle with police. The gunman, Aaron Alexis, a defense industry employee and former Navy Reservist, had complained of hearing voices and said in writings left behind that he was driven to kill by months of bombardment with electromagnetic waves.

The Associated Press contributed to this report.