The teenage gunman charged with unleashing one of America’s worst school shootings may have chosen Valentine’s Day to strike after the end of a relationship with a girlfriend.

Nikolas Cruz, 19, described by fellow pupils as a “weird and racist” gun-obsessed loner, has been charged with murdering 17 people at his former school in Parkland, an affluent suburb north of Miami, Florida.

It also emerged that Cruz had previously been treated at a mental health clinic, but was still able to legally purchase an AR-15 rifle, and large amounts of ammunition, passing a background check in February 2017.

Instead, the US president said he would “tackle the difficult issue of mental health” and that there had been “so many signs” that Cruz was “mentally disturbed.”

He added on Twitter: “Neighbours and classmates knew he was a big problem. Must always report such instances to authorities, again and again!” 

Jeff Sessions, the attorney general, said: “It cannot be denied that something dangerous and unhealthy is happening. We are going to take action. We must reverse these trends.”

And Rick Scott, the Florida governor, said: “If someone is mentally ill, they can’t have access to a gun.” Barack Obama, the former president, said on Twitter: “Until we can honestly say that we’re doing enough to keep them [our kids] safe from harm, including long overdue, common-sense gun-safety laws that most Americans want, then we have to change.”

We are grieving with Parkland. But we are not powerless. Caring for our kids is our first job. And until we can honestly say that we're doing enough to keep them safe from harm, including long overdue, common-sense gun safety laws that most Americans want, then we have to change.

— Barack Obama (@BarackObama) February 15, 2018

Survivors of the mass shooting also demanded action. David Hogg, a pupil who witnessed it, said: “We’re children. You guys are the adults. You need to take some action and play a role. Work together, come over your politics, and get something done.”

Another pupil named Sarah wrote on Twitter: “Multiple of my fellow classmates are dead, Do something instead of sending prayers. Prayers won’t fix this. But gun control will prevent it from happening again.”

Last night, Cruz made his first court appearance, dressed in an orange jumpsuit with his hands shackled to his waist. Asked to confirm his name he told the judge: “Yes Ma’am”. He was ordered to be held without bail.

It had earlier emerged that there had been a litany of missed warnings about Cruz.

Last year, Marjory Stoneman Douglas High School sent an email to its teachers telling them not to let Cruz on site if carrying a backpack, after bullets were found in his bag.

He was then expelled after fighting with the boyfriend of an ex-girlfriend, according to a fellow pupil. On Sept 24, a YouTube user calling themselves Nikolas Cruz posted a message saying: “I’m going to be a professional school shooter.”

It was left on the YouTube page of Ben Bennight, a bail bondsman in Mississippi, who immediately reported it to both YouTube and the FBI. The following day he was visited by two FBI agents, who took photographs of the post.

Mr Bennight next heard from the FBI on Wednesday, half an hour after Cruz was arrested. An agent left a message saying: “If you wouldn’t mind giving me a ring…” Robert Laskey, an FBI Special Agent, said the message in September had been investigated.

He said: “The FBI conducted database reviews, checks, but was unable to further identify the person who made the comment.” Cruz was born in New York but adopted as an infant, along with his brother, by Roger and Lynda Cruz. Roger Cruz died of a heart attack when Cruz was a child.

Lynda Cruz died of pneumonia on Nov 1 last year and he moved in with another family. 

Jim Lewis, a lawyer for the family with whom Cruz was living at the time of the shooting, said: “There was no indication that anything severe like this was wrong. Just a mildly troubled kid who’d lost his mother.”

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However, fellow pupils said they had always feared Cruz would “shoot the place up”. Dakota Mutchler, 17, said: “I think everyone had in their minds, if anybody was going to do it, it was going to be him.”