U.S.Capitol Police surround a car driven by Miriam Carey at the corner of Pennsylvania Avenue and 1st Street NW Thursday in this framegrab from Alhurra TV video.

New details emerged Friday about the final moments of an unarmed woman motorist who was shot to death by police outside the U.S. Capitol, but unanswered questions left room for debate on whether the shooting was justified.

Law enforcement officials who briefed NBC News on the case said that officers fired a total of 26 rounds at the driver, 34-year-old Miriam Carey of Stamford, Conn., who never left her vehicle.

Washington, D.C., police, who apparently were not involved in the shooting, are investigating whether U.S. Capitol Police and Secret Service officers complied with department policies on the use of deadly force in the killing of Carey on Thursday after a high-speed chase that began at the White House.

Most law enforcement agencies have policies that allow the use of deadly force only when officers have reason to believe that they are threatened by death or serious physical injury, raising questions about the gunfire that officers of the Secret Service and Capitol Police directed at her black Infiniti.

“There are definitely some major questions here," said Eugene O'Donnell, a professor of law at John Jay College, who has studied the issue of the use of deadly force. Among the key ones, he said, were what threat the officers believed Carey posed at the time they fired the shots and whether they were influenced by police radio transmissions reporting that there were "shots fired," causing them to conclude – wrongly, as it turned out -- that Carey was armed and dangerous.

David E. Haynes, an attorney who has sued police departments alleging brutality and wrongful shootings, agreed that Carey’s actions, while erratic, do not necessarily support a justifiable shooting.

“The exact facts are not known, particularly in the moments right before the shooting,” he said. “…Trying to breach a barrier, and not being able to, does not in itself justify lethal force."

Still, O'Donnell noted that both the Secret Service and Capitol Police have special missions to protect the president and members of Congress that give them more latitude in the use of deadly force than average police officers. The protocols for both agencies are not public, officials said.

Law enforcement sources tell NBC News that Carey had been suffering from depression and had delusions before the incident, believing that the federal government had her under surveillance and the President Barack Obama was communicating with her.

Related story: DC chase: Secret Service, Capitol Police radios can't communicate

Law enforcement officials, who spoke to NBC News on condition of anonymity, on Friday provided new details about Carey’s encounter with uniformed Secret Service officers outside the White House and the high-speed chase that ended with her death several minutes later near the U.S. Supreme Court.

Carey initially drove into the driveway leading to the White House at about 2:12 p.m. on Thursday, hit some temporary fencing, then backed up, striking a Secret Service officer, who suffered minor injuries.

Slideshow: Capitol car chase puts DC on edge Mark Wilson / Getty Images The U.S. Capitol was placed on lockdown following a car chase involving a woman who attempted to ram a security fence at the White House. Launch slideshow

Initial reports that the car tried to drive through a barrier onto the White House grounds were not correct, according to witnesses quoted by the Washington Post. Officers told the driver to stop, and placed a temporary barrier behind her car, and the driver backed into this fence and sped away, they said.

The law enforcement officials said the Secret Service officers pursued her at high speed on Pennsylvania Avenue – hitting 80 mph at one point -- headed southeast toward the Capitol.

The Secret Service confirmed Friday that the driver's car struck at least one Secret Service car during the pursuit, before being cornered near Garfield Circle and the Capitol Reflecting Pool a few minutes later. Video shot by the U.S.-funded Alhurra news network shows six officers with guns drawn approached the stopped car and attempted to order the driver from the vehicle. Carey responded by whipping the car around, scattering the officers.

Officers opened fire after Carey circumnavigated Garfield Circle, then exited onto Constitution Avenue, unleashing nine shots at the fleeing vehicle, the officials said.

At this point, the Alhurra video shows a Capitol Police car hitting a barrier that was raised after Carey’s car passed; the officer suffered only minor injuries in the crash.

The chase ended minutes later at the northeast corner of the Capitol grounds, near the U.S. Supreme Court building, when the Infiniti was stopped by a barrier near Second Street NE. The officials said she made a U-turn and became stuck on a median by a Capitol Police guard post, where she was shot dead. Seventeen shots were fired into the car at this point, they said.

The officials said Carey never left her vehicle, contradicting some early reports indicating she was shot as she exited the Infiniti, and confirmed that she was unarmed. The child in her car – later identified as her 1-year-old daughter -- was uninjured.

They also said that travel records reviewed by investigators indicated that Carey had driven straight to Washington, D.C., from Stamford on Thursday.

Despite the new details, numerous questions remain unanswered on the case, including, whether Carey had any communication with officers during any of the encounters, whether police knew a child was in her car, and whether Carey made any threatening movement during the final confrontation, on the northeast side of the Capitol.

A police trainer on the use of force said the shooting could be justified under the standards laid out by the U.S. Supreme Court "if the officers believe that she was going to still be a threat.”

“She's tried to run over several police officers, she's reckless and endangering the public -- we're still not sure why she tried to enter the White House in the first place -- it's reasonable to think that she's still a threat," said John Bostain. A former officer and instructor with the Calibre Press training company, Bostain presviously served as the senior instructor for use of force training programs at the Federal Law Enforcement Training Center in Georgia, though he was not speaking on behalf of FLETC.

Related story: Woman in DC chase may have thought Obama was stalking her, sources say

The key considerations under the court's Graham v. Connor decision, Bostain said, are these: Police are allowed to use force that is "objectively reasonable," based on the totality of the circumstances, from the perspective of a reasonable officer on the scene, at the moment force was used and without 20-20 hindsight.

Those circumstances include the heightened security in Washington after the terrorist acts of Sept. 11, 2001, the recent shooting at the Washington Navy Yard and other incidents. "To the average layman, a car that almost appears to have made a wrong turn into the White House -- it's a totally different perspective if you're a Secret Service officer guarding the White House,” he said. “... The car could contain a bomb. They have to stop them before they become a further danger to officers and the public."

From the opposite perspective, Haynes, the lawyer who has sued police, said that the key moment for a jury would be the time of the shooting.

"At this point a thorough investigation is necessary,” he said. “… Obviously law enforcement should be trained to de-escalate a situation and to avoid lethal force if at all possible. This lady was unarmed. … She had a baby in the car. Her behavior was erratic. … It appeared this was likely someone who was suffering from mental illness, but the officers don't have the luxury to know that. At the time the shots were fired, was she presenting a threat of serious bodily harm to the officers? Was she trying to flee? Or was she trying to injure or kill someone.”

Richard Esposito, Jonathan Dienst and Mike Brunker of NBC News contributed to this report.

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