A person close to the teenager accused of killing 17 people at a Florida high school warned the FBI she was concerned he would “get into a school and just shoot the place up,” according to a transcript of her call with a bureau tip line operator more than a month before the massacre.

The transcript, reviewed by The Wall Street Journal, provides chilling detail about the woman’s efforts on Jan. 5 to warn authorities about Nikolas Cruz’s propensity for violence and his troubled past. The caller’s identification was redacted...