Peter Langman was in bed Saturday morning when the phone rang. It was a reporter calling about Ali Sonbaly, the 18-year-old who went on a shooting rampage the night before at a mall in Munich.

Langman often gets phone calls after mass shootings. Though he’s a psychologist in private practice, he’s spent the last decade obsessively researching school shootings, writing two books widely admired by threat-assessment professionals and academics.

The reporter told Langman that Sonbaly had apparently been reading a German edition of Langman’s first book, “Why Kids Kill: Inside the minds of school shooters.”

“It’s incredibly disturbing,” Langman said in an interview. “It’s terrifying.”

The initial fear over Sonbaly’s attack was that it was terrorism. But as investigators looked closely at his life it turned out he was obsessed with mass shooters, committing his attack on the anniversary of Anders Breivik’s 2011 attack in Norway that left more than 70 dead.

I profiled Langman a few weeks ago. He is soft-spoken and deeply caring. The shootings have taken a toll on him. Sometimes, he has to back away from his research, which involves reading thousands of documents and police reports, often before bed.

But what brings him back is the good he knows he can do in getting his research out to schools, police and parents.

“People need this information,” he said. “That’s the point of doing it.”

In our conversations, Langman spoke of the care he takes to not include sensational videos or pictures on a website he maintains of his school shooting documents. He does the same in his books. He knows firsthand how much time mass shooters often spend researching their forebears.

“For me, it’s not about the violence,” Langman said. “It’s about what’s going on in their minds. That’s what we need to understand.”

What was Ali doing with Langman’s book?

The grimmest possible answer is the one Langman most fears: studying how to emulate and honor them. But Langman also wonders (or hopes) that Ali, who was in mental health treatment, was trying to get some insight into his own disturbed mind.

“We just don’t know yet,” Langman said.

This is not the first time one of Langman’s books has been found in possession of a shooter. Karl Pierson, an 18-year-old who killed a student at a Colorado high school in 2013, had a copy in his bedroom.

And Ali will probably not be the last.

“An author can’t control what someone does with their book,” Langman said. “You have to do the research and get it out there.”