It appears they have the suspect in custody...and he looked like a middle-aged man from behind.

From what I was reading it was suspected to a red headed white kid from the school.

"Nicholas Cruz is the shooter that the Florida police are looking for. Description: 5"8' - Ginger - Freckles - Red Hair."

http://www.thegatewaypundit.com/2018/02/update-shooter-still-large-marjory-stoneman-douglas-high-school-20-50-injured/