The suspect, who is now at a nearby hospital, was reportedly armed with an assault rifle and "countless magazines." Sen. Bill Nelson said the suspect "wore a gas mask and ... had smoke grenades."
In a multi-agency news conference, authorities placed a strong emphasis on caring for victims, family members and survivors of the tragedy.
Superintendent Robert Runcie said Marjory Stoneman Douglas High School will close for the remainder of the week. The school district said grief counseling services will be offered for students, family and staff members.
The Florida attorney general's office said it would pay for victims' funeral expenses. The state will also cover counseling services for survivors.
Deputies responded to reports of a shooting shortly before 3 p.m. local time at the high school in Parkland. The 19-year-old suspect was a student at the school before being expelled, according to the Broward County Sheriff's Office.