The FBI admitted on Friday it failed to investigate a specific report in January that Nikolas Cruz could be plotting a school shooting.

The agency said the tip should have been investigated thoroughly because it was a “potential threat to life”. Cruz was arrested on Wednesday and has since been charged with murdering 17 people at a high school this week.

On 5 January, a tipster who was close to Cruz called the FBI and provided information about Cruz’s guns, desire to kill people, erratic behavior and disturbing social media posts. The FBI says the caller expressed concerns Cruz could attack a school.

Jeff Sessions, the US attorney general, issued a curt statement after the FBI admission saying that warnings signs had been missed with “tragic consequences”. He said: “We must do better.”

Sessions said he had ordered a department of justice review of how the FBI and DOJ respond to indications of potential violence. The review would consult family members, mental health officials, school officials and local law enforcers, he said.

Also on Friday, the Associated Press reported that Cruz took part in an air-rifle marksmanship program supported by a grant from the National Rifle Association Foundation, part of a multimillion-dollar effort by the gun group to support youth shooting clubs. He was wearing a maroon shirt with the logo from the Army Junior Reserve Officer Training Corps [JROTC] program when he was arrested.

Records show that the Stoneman Douglas JROTC program received $10,827 in non-cash assistance from the NRA’s fundraising and charitable arm in 2016, when Cruz was on the squad.

The news came on Friday as prosecutors, mental health experts and lawmakers grapple with the aftermath of the mass shooting in Parkland, Florida, one of the dilemmas confronting them is how a clearly disturbed teenager, described by his lawyer as a “broken child”, could have allegedly carried out such a coolly premeditated slaughter of 17 people.

Play Video 1:26 'He shot through my door': survivors recount Florida high school shooting - video

Nikolas Cruz’s arrest sheet records that by his own confession he brought several loaded magazines hidden in a backpack into Marjory Stoneman Douglas high school on Wednesday, enough to sustain a whirlwind of almost 150 shots discharged over three floors of the building. His carnage completed in under six minutes, he calmly followed through with his plan to merge into the crowd of fleeing students and wander off undetected, stopping in at a Walmart, a Subway sandwich shop and a branch of McDonald’s before being apprehended about an hour later.

In stark contrast to a portrait of a calculated murderer is the depiction of Cruz presented by his lawyers at his first court appearance on Thursday, backed up by numerous accounts from neighbors and school peers and the evidence of his own social media posts. This 19-year-old was deeply troubled, potentially autistic, according to his defense team, and devastated by the recent death of his mother Lynda who had adopted him when he was two.

“He was just lost after that,” said Gordon Weekes, one of his public defenders, referring to Cruz’s mother’s death from pneumonia last November.

In the hindsight that routinely follows mass shootings, there were plenty of warning signs. Police records obtained by CNN show that police were called to his house 39 times over seven years.

In that time, Cruz revealed himself to have certain obsessions. He liked to hurt animals and used a BB gun to shoot squirrels, frogs, even neighbors’ chickens. He would drive sticks down rabbit warrens in the hope of skewering the inhabitants.

His other fixation was guns. The AR-15 that the authorities said he used to carry out the massacre was kept in a gun box in his bedroom to which he had his own key.

He also had a collection of air rifles, which he displayed sprawled out on his bed in an Instagram post. A video captured by a neighbor showed him shooting a BB gun in his back yard; he was wearing only boxer shorts and one of the Make America Great Again caps associated with Donald Trump’s campaign slogan.

Trouble followed Cruz into school. He was expelled last year from Stoneman Douglas –where he so fatefully returned this week – for reasons that remain unclear but could have included violence and bringing in a knife.

Play Video 2:26 17 confirmed dead in 'horrific' attack on Florida high school – video report

Given this litany, investigators are under pressure to explain how warning signs were missed. Despite strong signals,including evidence that he was treated for psychological difficulties last year, why was he allowed to fall off the radar of mental health services?

Even more relevant, perhaps, is the YouTube post from a user named Nikolas Cruz expressing a desire to become “a professional school shooter”, which the FBI probed without conclusion. How did the suspect pass a federal background check that enabled him to buy an AR-15 at Sunrise Tactical Supply gun shop in Coral Springs, entirely legally?

Such questions go to the heart of the Parkland tragedy – for the 17 victims of the rampage, their families and the scores of other students and teachers traumatized by the massacre. And for Cruz himself, a teenager for whom help was never close enough at hand, who now faces the prospect of dying in prison should he succeed in avoiding the death penalty.

The Associated Press contributed reporting