Flight attendants initially told passengers to stay in their seats and not evacuate, officials said.
The cockpit told flight attendants to wait while crew communicated with air traffic controllers, Hersman told reporters. Evacuation began about 90 seconds after the plane stopped moving, when a senior flight attendant spotted flames on the right side of the plane.
"We don't know what the pilots were thinking," Hersman said. Some crews wait to evacuate until first responders arrive, she said, and the pilots may not have realized that flames were nearing the cabin.