The FBI was warned last month that Nikolas Cruz was an armed psycho who might shoot up a school — but it didn’t bother investigating, the agency admitted Friday.

“A person close to” Cruz called the agency’s tipline on Jan. 5 and reported the 19-year-old had a “desire to kill people, erratic behavior, and disturbing social media posts” and there was “potential of him conducting a school shooting,” the FBI said in a statement.

But the agency said it failed to pass on any of that information to its Miami field office, even thought its own protocols say he “should have been assessed as a potential threat to life.”

A little over a month later, on Wednesday, Cruz allegedly gunned down 17 people at his former high school.