When two Palo Alto police officers shot and killed William David Raff, 31, outside a Forest Avenue home on Dec. 25, they instantly triggered speculation among the victim's family, friends and the broader community about whether the shooting was just -- and justified.

The question has become increasingly common across the United States in recent years, with high-profile police shootings in Ferguson, Missouri; New York City; Cleveland; Baltimore; and Chicago prompting conversations about race and policing. Just this week, Chicago Mayor Rahm Emanuel ordered a fresh look at how police officers handle calls involving people with mental health problems -- a decision that was prompted by the Dec. 26 fatal shooting of 19-year-old Quintonio Legrier, and his neighbor, Bettie Jones, 55, by a police officer.

The shooting has added more fuel to public outrage over last year's killing of LaQuan McDonald, 17, by a Chicago police officer, an incident that led to the ousting of the city's police superintendent and prompted citizen calls for Emanuel's resignation.

Public concerns about police tactics have also surfaced periodically in Palo Alto, where every incident involving a fired Taser gets scrutinized by an independent police auditor and where the police chief was forced to resign in 2008 after making comments that many in the community perceived as a tacit endorsement of racial profiling.

In 2012, an officer was ordered to undergo additional training for firing a Taser at a 16-year-old bicyclist in violation of department policy. The department also had to pay a $35,000 settlement after a 2008 incident in which officers allegedly lured a man from his van and stunned him with a Taser.

Fatal shootings, meanwhile, remain an extremely rare phenomenon in Palo Alto. Before officers shot and killed Raff, who reportedly charged at them with a table knife, the city hadn't had a police-involved shooting in more than a decade. The last time a person was shot and killed by a Palo Alto officer was in March 2002, when 20-year-old Pedro Calderon was shot and killed after allegedly trying to flee the police in a stolen BMW.

Palo Alto Officer Jessica Perryman and Stanford police Deputy Jeff Bell each fired at Calderon after he allegedly pinned Perryman down between his car and her own, injuring her.

Though that shooting prompted a protest by East Palo Alto residents who knew Calderon, an investigation ultimately cleared Perryman and Bell of any wrongdoing.

Now, Palo Alto police and the Santa Clara County District Attorney's Office are investigating whether deadly force used by the two officers was justified against Raff, who reportedly suffered from schizoaffective disorder and lived in the group home operated by La Selva, an organization affiliated with Momentum For Mental Health.

At the center of the investigation is the question: Did officers Nicholas Enberg and Zachary Wicht believe that Raff posed an imminent threat to themselves or others in the moments before they fired their pistols?

According to the police, video footage from the incident shows Raff charging at the officers with a knife just before Enberg and Wicht fired their pistols at him. The police department's existing policy allows an officer to use deadly force to protect himself/herself or someone else from what the officer "reasonably believes would be an imminent threat of death or serious bodily injury."

The policy also deems deadly force to be justified when an officer is trying to stop a fleeing subject who the officer has probable cause to "believe that the person has committed, or intends to commit, a felony involving the infliction or threatened infliction of serious bodily injury or death, and the officer reasonably believes that there is an imminent risk of serious bodily injury or death to any other person if the subject is not immediately apprehended."

The policy specifies that under such circumstances, "a verbal warning should precede the use of deadly force, where feasible."

Under the policy, deadly force can be justified even if the suspect isn't pointing a weapon at someone. Imminent danger, in this case, "does not mean immediate or instantaneous," the policy states. It could apply, for example, if an officer reasonably believes that the person has a weapon or is attempting to access one and that he or she "intends to use it against the officer or another."

Imminent danger can also exist if the person "is capable of causing serious bodily injury or death without a weapon and it is reasonable to believe the person intends to do so."

Though the investigation into the Dec. 25 shooting is still in its early stages, police said officers responded to the group home at 652 Forest Ave. for a "false emergency" call in which Raff reported that a person at the residence is "really violent" and provided the name of that person. Police said there was no one by that name in the residence.

The investigation indicated that Raff may have been waiting outside for the police.

When the officers got to the scene, Raff allegedly "charged at them in the street while armed with a knife," police said in a statement. Enberg and Wicht then fired their guns at Raff. Officers reportedly provided first aid to Raff before paramedics arrived and transported him to the hospital, where he later died.

William Raff's father, Garold Raff, told the Weekly on Monday that he believes the shooting was "unjust" and that officers used excessive force on his son. But under the department's policy, officers are allowed to use "reasonable force" to make an arrest, prevent escape or overcome resistance.

The department's policy includes 17 different factors that officers can consider in determining whether to apply force and whether the force is reasonable. These include the immediacy and severity of the threat to officers or others; the conduct of the individual (as "reasonably perceived by the officer at the time"); the person's "mental state or capacity"; proximity to weapons; and whether the person appears to be "resisting, attempting to evade arrest by flight or is attacking an officer."

Enberg and Wicht have been placed on paid administrative leave, as is standard with any officer-involved shooting, the police department said in a news release.

Once the police investigation is completed, the Santa Clara County District Attorney's office will review all reports, videos, interview recordings, the autopsy report and other materials to decide if any criminal charges should be filed against the officers, Assistant District Attorney James Gibbons-Shapiro said in an email.

But the public should not expect a speedy turnaround.

"This particular case is in its very early stages, as we do not expect a completed autopsy report for some weeks and because there are more witnesses to interview, among other things," he said.

If no charges are filed, the DA's office will release a detailed report, including how the DA investigators reached their conclusions. The report is usually released within 60 days of completion, but sometimes additional steps must be taken before there is a decision, he said.

Staff Writer Sue Dremann contributed reporting to this story.