United States President Donald Trump has suggested the suspect in the Florida shooting that killed 17 people is possibly "mentally disturbed"

The ex-pupil, identified as Nikolas Cruz, 19, allegedly walked into the Marjory Stoneman Douglas High School in Parkland on Wednesday shortly before classes ended and opened fire on students and teacher.

Cruz was armed with an AR-15-style rifle and had multiple magazines of ammunition when he surrendered to officers in a nearby residential area, police said. He loved guns and was expelled for unspecified disciplinary reasons, police and former classmates said.

In a tweet on Thursday, Trump suggested "signs" had been missed.

"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior. Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"

The comments are similar to his remarks after November's Texas church shooting - where at least 26 people were killed and 20 were injured - which he said from a "mental health" problem, not a "guns situation".

His words are at odds with one of his first actions as President.

Behind closed doors and to cheers from the National Rifle Association, he rolled back an Obama-era regulation that would have made it harder for people with histories of mental illness to buy guns.

In doing so, he prevented an estimated 75,000 names of social security recipients with mental illnesses, people who need help managing their finances, from being added to the national background check database.

Trumps latest excuse is problematic - and not just because of his hypocrisy.

The vast majority of those with mental illness are non-violent and only 3 to 5 per cent of violent acts can be attributed to those living with a serious mental illness.

Peter Ambler, the executive director of fun control group Giffords, told NBC News:

Trump is wrong - study after study show that stronger gun laws can save lives - and a hypocrite of the worse kind. One of his first actions as president trashed a new regulation that would have prevented potentially irresponsible and mentally incompetent people from being able to buy guns.

Ambler maintained that Trump's comments do not suggest he will rethink rolling back Obama's gun-check rule, which received its share of criticism for singling out the disabled.

He added:

Blaming mental health is a tactic straight out of the gun lobby's playbook that's meant to paralyse Congress. Donald Trump's goal is to make people think our leaders don't have the power to prevent gun violence.