NEW YORK (NYTIMES) - Last spring, as the end of the academic year approached at Susquehanna Valley High School outside Binghamton, New York, students were asked for a school project about their plans after graduation.

Payton Gendron, a senior, said he wanted to commit a murder-suicide, according to a law enforcement official briefed on the matter.

He claimed to be joking, the official said. But the state police were summoned to investigate and took Gendron, then 17, into custody on June 8 under a state mental health law, police officials said Sunday (May 15).

He had a psychiatric evaluation in a hospital but was released within a couple of days, the officials said. Two weeks later, Gendron graduated and fell off investigators' radar.

On Saturday, he resurfaced 200 miles (320km) away in Buffalo, New York, where authorities say he opened fire at a supermarket in a predominantly Black area, killing 10 people and wounding three others in one of the deadliest racist massacres in recent US history.

After his rampage, Gendron put his gun to his neck. But two officers persuaded him to drop his weapon and surrender.

He was charged Saturday with first-degree murder, and as he awaited his fate in jail, investigators were sifting through his past to piece together how he transformed from a quiet student to an accused killer without drawing more serious scrutiny.

New York state has what is known as a red flag law, under which people found to be a danger can be forced to surrender their guns, but no one tried to invoke it against Gendron. State police said he had not named a specific target in his threat to kill someone.

But the episode came after what former classmates said was a pattern of increasingly bizarre behaviour by Gendron. Two former classmates said he showed up to class in hazmat gear after pandemic restrictions were lifted in 2020.

"He wore the entire suit, boots, gloves, everything," Nathan Twitchell, 19, said as he stood on his porch in Binghamton, shaking his head. "Everyone was just staring at him."

That was one of the few times students saw Gendron, said Cassaundra Williams, another student at the high school. Williams, 19, said Gendron favoured online coursework even as his classmates returned to campus.

"He was always very quiet and never much said anything," said Williams, who added that Gendron was book smart but had grown more reclusive over the years since she met him in elementary school.

"We were just so shocked. We can't even wrap our heads around it still," she said.

Gendron's mother did not respond to a message left Sunday afternoon. Nor did the lawyer who represented Gendron at his arraignment, Brian Parker.

Williams said the last time she had seen Gendron was at graduation. She said she was shocked when a friend texted her after the shooting Saturday to tell her that Gendron had been arrested.

"He was just a quiet, smart kid that I wouldn't think would be able to do anything like what he did yesterday," said Twitchell. "It just blows my mind."

Kolton Gardner, 18, who attended middle school and high school with Gendron, described him as "definitely a little bit of an outcast."

"He just wasn't that social," Gardner said. "I knew he had an interest in guns, but where we grew up that wasn't uncommon. That's just kind of the thing in rural New York, people like guns."

One of many unanswered questions posed by Gendron's rampage is why his grim response about his post-graduation plans did not lead to further intervention beyond the mental-health exam.

Under New York state's red flag law, enacted in 2019, anyone who believes that someone may be a threat to themselves or others can ask a judge to issue an "extreme risk protection order" that prevents the person from purchasing or possessing a firearm. The law is not used often.

The law enforcement official who had been briefed on the school project said that in New York, there are hundreds of school threats called in each year, and that in each case, authorities interview the students and their parents to determine whether students have access to guns. Authorities then try to make a reasoned call.

In any case, Gendron was not on any red-flag list when he entered Vintage Firearms in Endicott, New York, and bought the Bushmaster semi-automatic rifle that police say he used in the shooting.

Robert Donald, the owner of the store, confirmed that his records showed he had sold the gun to Gendron, but said he did not remember the young man at all, even though he said he sells only a half-dozen or so of this type of gun in a year.

Gendron wrote that he modified the gun with his father's power drill, using a parts kit that retails for US$60 (S$84).

Donald said that when he sold Gendron the firearm, its design complied with state law banning military-style features.

"Even with all of those safety features on it - which is the only way I sell it - any gun can be easily modified if you really want to do it," he said.