President Donald Trump in his tweets on Thursday called the suspected gunman in the massacre of 17 people at a high school in southern Florida “mentally disturbed”.

The 19-year-old, identified as Nikolas Cruz, was due in court on Thursday.

He was arrested after Wednesday’s shooting spree in Parkland, Florida, at Marjory Stoneman Douglas High School, where the former student had previously been expelled for what school officials said were “disciplinary reasons.”

“So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behaviour,” Trump tweeted.

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

The White House said Trump would speak publicly about the “tragic shooting” at 11 am (1600 GMT).

Authorities said 15 people were hospitalized with injuries suffered in the attack, which occurred late in the school day.

Police said the gunman opened fire with an AR-15 rifle after setting off a fire alarm in the school, 75 kilometres north of Miami.