A Virginia public school student suggested he might carry out an ethnic cleansing at his school. Officials were alarmed, but they weren’t sure whether he was serious. They convened a team to assess the threat and notified the police, who found an unsecured loaded semiautomatic pistol in a search of his home.

An investigation revealed online messages with a friend in another state considering a similar act — acts that police then thwarted, according to Dewey Cornell, a scholar who keeps track of these incidents but does not give out time or location information to protect the confidentiality of the districts and students.

One of the stickiest dilemmas that counselors and teachers face is how to know which students are poised to commit a horrid act of violence and which ones just need help, even urgently. Every day, they walk a terrifying tightrope: Removing every student who shows disturbing behaviors would amount to a huge overreaction. Missing just one true killer could be a tragic underreaction.

“It boils down to a judgment call,” said Melissa Reeves, former president of the National Association of School Psychologists. “We don’t have a cookbook that says if this situation happens, do this.”

Increasingly, school officials rely on sophisticated threat detection systems designed to head off horrific mass shootings like the one last week in Oxford, Mich. When officials encounter concerns, they might look at social media posts, search a student’s locker or desk and examine academic and disciplinary records. Officials might investigate the student’s access to weapons, recent stressful events in the student’s life, evidence of depression or suicidal thoughts, evidence of planning an attack and consistency of a student’s statements and actions.

In Oxford, 15-year-old Ethan Crumbley came to the attention of guidance counselors, who sent him back to class. Later that day, he is alleged to have opened fire, killing four students and injuring seven others.

Many academics, school officials and law enforcement agencies suggest decisions like this be made by a much larger, multidisciplinary team including administrators, school-based police officers and mental health professionals such as school counselors, psychologists and social workers.

“It doesn’t rest on one person’s shoulders, or one or two,” said Michele Gay, whose 7-year-old daughter Josephine was killed in the mass shooting at Sandy Hook Elementary School in 2012. Gay now runs a nonprofit organization that brings threat assessment training to U.S. schools. “We know that it works, we know it saves lives.”

The U.S. Secret Service has long wrestled with this problem as it has sought to evaluate people who might attack the president or other protected officials. In 1999, after a mass shooting at Columbine High School outside Denver left 13 people dead, the agency realized its methods could be used to evaluate threats in schools, too, and began sharing its tools with educators. By 2018, the Secret Service’s National Threat Assessment Center had published a 32-page guide that recommended multidisciplinary threat assessment teams, with a set of criteria for intervention. The FBI also sees threat assessment as a valuable tool in trying to prevent mass school shootings.

“You can’t catch someone falling from a building if you’re the only one holding the net,” said Katherine Schweit, a former FBI special agent and author of the book, “Stop the Killing: How to End the Mass Shooting Crisis.”

After the 2007 mass shooting at Virginia Tech, in which 32 people were killed, Virginia became the first state to mandate threat assessment teams, taking the approach recommended by the Secret Service. The requirement was imposed first on universities and, in 2013, on K-12 schools. Now at least 11 states require schools to put these teams in place. One study found that by 2017, about half of American secondary schools and more than 40 percent of primary schools used threat assessment teams.

The Oxford Community School District did not respond to a question about whether it uses a threat assessment system.

In Oxford, on Nov. 29, a teacher saw Crumbley searching online for ammunition. The next day, a second teacher found him with a disturbing drawing of a bloody figure, a gun, a bullet and words including “the thoughts won’t stop” and “help me.” Both teachers reported the incidents. Crumbley was sent to the guidance counselor’s office, where he told counselors the art was part of a video game he was designing, the district said.

After his parents resisted sending their son home for the day, counselors ordered them to find counseling for him within 48 hours and said the school would notify child protective services if they didn’t. He was sent back to class, and that afternoon, according to authorities, he launched his attack.

Cornell, an education professor at the University of Virginia, has developed a widely used threat assessment tool and said that case should have been handled by a threat assessment team. He said any counselor concerned enough to mandate counseling would have been concerned enough to bring the case to such a team, if one were in place.

“Without all the facts and context, it is not possible to judge what the Oxford school authorities should or should not have done, but it appears to have been a preventable event, as are most school shootings,” he said.

His system, first developed in 2001, has been subject to six controlled studies to test its effectiveness, he said. It includes a decision tree that asks school officials to interview the student and other witnesses. School officials are to consider questions, such as whether the student communicated an intent to harm others, whether he or she had offered an explanation or apology, whether there is a way to resolve the conflict, whether mental health services are needed and whether law enforcement should be involved.

One challenge for administrators is knowing when students and their parents are telling the truth. Tracy Ogren, a school psychologist with Robbinsdale Area Schools in suburban Minneapolis, which uses Cornell’s system, often asks to see their social media accounts. If they refuse, that “tells us something, too,” she said. In Oxford, the parents did not tell the school that their son had access to a gun, and officials said they are reviewing social media posts that appear to show Crumbley cradling a Sig Sauer 9mm handgun — the same make of the weapon used in the shooting — in the days leading up to the shooting.

Many experts say students are far more likely than adults to see warning signs early. Teachers are well positioned to spot disturbing ideas and emotions that might surface in class assignments such as artwork or essays, said Robyn Lady, a director of student services with Fairfax County Public Schools in Virginia, but teens have an inside window to social media posts, where they are candid about thoughts and plans. Lady holds yearly trainings for students on how to detect and report possible threats.

Reeves, who trains school systems nationwide on using threat detection systems, points out that they still don’t always work perfectly.

Things become “fuzzy,” Reeves said, when a student is not targeting a specific person, or when the student does not appear to pose an imminent risk, meaning no evidence suggests that the student is “going to carry out their plans right now,” she said. It’s not always clear what to do. She said it can also be hard to quickly discern whether someone is lying while gathering information to assess the seriousness of a threat, particularly under pressure. She said the best approach is to gather as much information as possible from as many sources as possible, beyond children and their parents.

“Sometimes we need to interview other students, and we do that confidentially,” Reeves said. “Sometimes the kids will know if there is a gun in the house, for example, because ‘I’ve been there and I’ve seen it’ or ‘So-and-so told me they just got one.'”

Another problem comes when schools are overly deferential with parents, said Kevin Ozar, a middle school American history and English language arts teacher at Farmington STEAM Academy outside Detroit. He suspects that’s partly due to fears that parents will pull their children from the district, which means the district loses thousands of dollars in state funding.

“I’ve seen sometimes districts get nervous upsetting the parents who they view as clients,” Ozar said.

He pointed to another challenge involving race and discipline. Schools have historically delivered tougher discipline to students of color than White students for the same infractions. Now, many districts are trying to correct the problem by holding off on disciplinary measures. Ozar said this sometimes leads to bad outcomes.

“I had a parent crying literal tears that they wish we would discipline their child more, but our building and district didn’t feel we could because of the demographics they fit into on paper,” he said. “The discipline is so messy right now.”

Cornell, who is hired by districts to train them on his threat assessment tools, said the system is mostly likely to identify cases in which a student is being bullied or harassed and needs professional attention. The program helps to avoid overreactions, he said, and replaces discipline with help.

In other cases, he said, there are warning signs about violence, but it’s hard to know if a violent act actually would have occurred. Once in a while, though, he hears of a case in which the system appears to have stopped a true tragedy.

A few years ago, Cornell said, a high school student in Virginia who had been bullied and was in conflict with teachers, made a threat to carry out a school shooting. It was deemed serious by the threat assessment team, and he was temporarily removed from the school.

When he returned, he was required to check in with a school official daily. After the 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Fla., where 17 people were killed, the boy thanked his school psychologist for the help he has received.

“That could have been me,” he said.