A woman who knew the teenager accused of shooting 17 people to death at a Florida high school last week told the FBI last month that Nikolas Cruz possessed an arsenal of weapons and ammunition, and she worried he might be “getting into a school and just shooting the place up.”

“I know he’s going to explode,” the woman said in a call to the FBI’s tip hotline on Jan. 5, according to a transcript of the call obtained by The New York Times.

The details of the call were first reported Friday by The Wall Street Journal.

The tip came in more than a month before the Feb. 14 massacre at Marjory Stoneman Douglas High School in Parkland. It was the clearest warning sign ignored by the authorities that Cruz, who had a long history of troubling behavior, represented a dangerous threat to others.

Over the course of the call, which lasted more than 13 minutes, the tipster warned the FBI that Cruz had been adrift since his mother’s death in November. The tipster provided four Instagram accounts for Cruz, which she said showed photos of sliced up animals and the firearms he had amassed. The caller, whose name was redacted on the transcript, said Cruz had used money from a life insurance policy after his mother’s death to purchase the weapons.

“If you go onto his Instagram pages, you’ll see all the guns,” the woman said.

Before calling the FBI, the woman telephoned law enforcement officials in Parkland, worried that Cruz might kill himself. But she didn’t hear back from them and became increasingly alarmed after she said Cruz posted online that “he wants to kill people.”

The Broward County Sheriff’s Office, which staffs the Parkland police station, received a call in November from a caller raising similar concerns about Cruz: that he was collecting guns and knives, might kill himself and “could be a school shooter in the making.” The caller was in Massachusetts, the Sheriff’s Office said in a summary of the call released Thursday.

Two deputies have been placed on restricted duty while the office investigates how two calls about Cruz — the one in November and an earlier one in 2016 — were mishandled.