When a young man well-known to police and neighbors for bizarre and scary behavior suddenly stabbed a former schoolmate to death in Winchester last month, many people asked why someone so seemingly dangerous was allowed to roam freely.

It is a natural question. Also one that, despite the horrific tragedy, has no easy answer.

One reason to pause: So much is still unknown about the case of Jeffrey Yao.

Virtually nothing has been publicly disclosed about whether Yao gave any signs that his apparent mental illness was worsening in the months immediately before he used a kitchen knife to end the life of 22-year-old Deane Kenny Stryker.

Should he have been committed to a psychiatric hospital before it came to that? It’s possible that he had been. It’s not now known what actions were taken after the five occasions between March 2013 and last September when police took him to Winchester Hospital. He may well have spent substantial time off the streets and in treatment. Officials so far haven’t said.

And then there is the tricky matter of discerning when someone is sufficiently dangerous, to themselves or others, to detain them — against their will, if need be.

The behavior by 23-year-old Yao that drew police attention included property damage, an attempted break-in, threats of suicide, yelling, and staring. But strange and annoying behavior doesn’t rise to the level of justifying involuntary commitment, said Paul Zeizel, a Brookline-based forensic psychologist who evaluates criminal defendants, including people who have killed.

Additionally, a person cannot be held involuntarily unless there’s an “imminent’’ risk of violence. “It’s really hard to say, ‘This guy’s going to be really dangerous in three months.’ We don’t know,’’ he said.

The most dangerous people are those who have paranoid hallucinations in which they are commanded to take actions, especially if they also abuse drugs, Zeizel said. Patients never suddenly “snap,’’ he said, but their heightening paranoia may not always be evident to those around them.

“It’s a very, very tragic case,’’ Zeizel said of Yao. “These cases do happen and they will continue to happen because we have a free society.’’ There are not enough places to confine all the mentally ill people, and besides, he said, mentally ill people are not the ones responsible for most violence. “Ninety-five percent of murders are committed by people who don’t have mental illness,’’ Zeizel said.

Police said that Yao came willingly when they took him to the hospital, so he may have agreed to hospital stays without the question of commitment coming up. Neighbors reported periods of time when he was not seen, and they surmised he was in the hospital.

The most recent time was in September. Yao was arrested for attempting to break into a neighbor’s house, and he wasn’t seen in the area for a time afterward.

Notably, there were no reports of trouble between September and the stabbing.

If police had brought Yao to the hospital during that time, based on his history it’s likely he would have been quickly admitted, involuntarily if need be, said Dr. Melisa Lai-Becker, president of the Massachusetts College of Emergency Physicians. But there is no record of that happening.

Assuming Yao was hospitalized in September, Lai-Becker questioned whether there was an “airtight plan’’ for his care and monitoring after discharge.

Mental health professionals say the focus on hospitalization misses the more relevant question: What care did Yao and his parents receive at home?

“Just because someone’s mentally ill doesn’t mean they need to be locked up,’’ said Susan Rudman, a Salem psychologist who sometimes works with the courts. “They need to be monitored. They need to have a strong network in the community.’’

Dr. Don Condie, past president of the Massachusetts Psychiatric Society, said that medications effectively quash homicidal tendencies — as long as the patient keeps taking them. It’s not known whether Yao was prescribed medication, or if so, whether he had stopped taking it. But Condie said, “I’ve never met anyone who, when on medication to treat mental illness, was still expressing homicidal ideation.’’

The most effective way to prevent violence, Zeizel said, is community engagement: a system in which clinicians are assigned to patients as they are discharged from the hospital, to coordinate their living arrangements and medical appointments.

Even when patients are committed, psychiatric hospitals are a temporary stopgap. They keep patients safe for a time as they weather a crisis, and help stabilize them and adjust their medications. Hospitals typically don’t cure severe mental illnesses, which tend to be lifelong and chronic. So patients will eventually return home and become the responsibility of their families and community.

Massachusetts is among only a handful of states that don’t have a provision for mandating outpatient treatment, a possible solution for people who cycle in and out of hospitals and don’t always follow their treatment plans.

Decades ago, people with an array of mental conditions were confined to asylums, often for a lifetime. In the 1960s, after reports of horrific conditions, Massachusetts and other states began shutting down those institutions with the idea of providing services in the community. Those services, if they were ever adequate, have eroded over the years in many areas.

While states eliminated mental institutions, they kept laws on the books allowing for civil commitment of people deemed dangerous to themselves or others. The question comes up surprisingly often — but the danger is usually that patients will harm themselves, not others.

Under Massachusetts law, a police officer who fears a person may harm himself or herself or others due to mental illness may bring the person to a hospital or mental health facility for an examination, even if the person has not committed a crime. Police have the authority to restrain those who do not come willingly.

“Many of these are not really police issues, but we are the people thrust into addressing them,’’ said Everett Police Chief Steven A. Mazzie.

His department has fostered relationships with local mental health providers, Mazzie said, and has increased training for officers to teach them the best procedures to handle people with mental illness in the community. “Many of these people don’t belong in police lockup,’’ he said.

People brought by the police to the hospital are under no obligation to participate in an examination, said Mark Larsen, director of mental health litigation for the Committee for Public Counsel Services.

Hospitals see such people frequently. The emergency department at Everett Hospital, where Lai-Becker works, receives at least three patients a day who might be dangerous to themselves or others; about three-quarters are sick enough to be sent to an inpatient psychiatric unit, the vast majority because they are suicidal.

Once they arrive at the facility, Lai-Becker said, most willingly sign themselves in. If they refuse, the hospital can invoke the state law that enables it to hold the patient for three days.

Condie, who made decisions on dangerousness for more than 20 years, said he always tried to persuade people to sign themselves in. “You need an alliance with this person. If you’re going to be a good doctor and treat them, you need to meet them where they are,’’ he said.

In making difficult decisions about whether a patient should be considered dangerous, Condie said he would add up the risk factors. People who are male, use drugs, have acted aggressively, and have been violent in the past are all more likely to become violent.

Dr. Elliot Pittel, a child and adolescent psychiatrist in Boston and Leominster, said he also considers the kind of home the person will return to if released. Does the person have access to weapons? Does the person have a stable place to live, a family, a therapist? “It’s not just, is this somebody who’s raging and agitated when you see them,’’ he said. “They could be sitting there calmly and say, ‘My family’s overreacting.’ . . . Then you have to hear from all parties.’’

After the three-day commitment to a psychiatric unit, if the patient doesn’t want to stay but doctors believe the patient should, the matter goes to court. The person is entitled to representation by a lawyer and the decision to commit someone is made by a judge, with doctors appearing as witnesses.

The hospital must prove beyond a reasonable doubt that the person is mentally ill and would likely harm themselves or others if not hospitalized, or would be unable to safely care for themselves. And it must show that less restrictive options would not be appropriate for the person.

A person may be civilly committed for up to six months. If they get better, they can be released before the end of the commitment. If they don’t get better, the facility can apply to the court for additional commitment terms, up to one year at a time.

Winchester Police Chief Peter MacDonnell has said he wants his department’s long record of interactions with Yao to “create some kind of dialogue on mental health in our society.’’

“This situation shows maybe we need to do more,’’ he said.

But with current laws and current understanding of mental illness, “doing more’’ is unlikely to mean locking people up more readily.

Cheri Andes, executive director of the National Alliance on Mental Illness of Massachusetts, said that even people with severe illness can be cared for in their home communities, and won’t be violent if they do get care.

Condie worries that shocking episodes like the fatal stabbing in Winchester will further distort public perceptions of mental illness.

“Most dangerous people are not mentally ill. Most mentally ill people are not at all dangerous,’’ he said. They are more likely to be victims of crime than perpetrators.

“When people are scared because a horrible murder has happened,’’ Condie said, “it’s much more difficult for them to empathize with the fact that most people with mental illness are picked on and bullied and vulnerable.’’

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her on Twitter @felicejfreyer. Mark Arsenault can be reached at mark.arsenault@globe.com. Follow him on Twitter @bostonglobemark