CNN) — The cockpit voice recorder of Asiana Airlines Flight 214 appears to show the pilots tried to abort the landing just 1.5 seconds before it crashed at San Francisco International Airport, the head of the National Transportation Safety Board said Sunday.

The pilots appear to have increased speed 7 seconds before impact, and they then "called to initiate a go-around 1.5 seconds to impact," Deborah Hersman said.

The NTSB's preliminary assessment of the plane's cockpit and flight data recorders show the flight was coming in too slow. But when asked if pilot error was to blame, Hersman said the crash landing was still under investigation.