17 people killed in rampage at Florida high school

Gunman 'wore gas mask' and used an AR-15 assault rifle

Suspect had been expelled for disciplinary reasons

Shooting is 18th in a US school this year so far

Suspect Nikolas Cruz 'had been flagged as a threat' and 'was crazy about guns'

'The new normal': With a school shooting every 60 hours, US accepts grim reality​

A former student at a Florida high school returned to his campus with deadly intent on Wednesday, reportedly donning a gas mask and killing at least 17 people with an AR-15 assault rifle.

Pupils at Marjory Stoneman Douglas High School in Parkland, 30 miles north of Fort Lauderdale, hid under desks and cowered in cupboards as the gunfire rang out.

The alleged gunman, named as Nikolas Cruz, 19, had previously attended the high school and was expelled for unspecified disciplinary reasons.