NEW YORK  The captain of a Southwest Airlines plane that landed on a collapsing nose gear at LaGuardia Airport took control from the first officer just 400 feet from the ground, the National Transportation Safety Board said Tuesday.

It's unusual for a co-pilot or captain to take over a plane that's landing when it's so close to the tarmac unless there are "profound" safety issues, a private aviation expert said.

Sixteen people were injured in the July 22 hard landing, which the NTSB is still investigating.

The federal agency, which investigates every U.S. civil aviation accident and significant accidents involving other types of transportation, on Tuesday issued an update report that offers a glimpse into what happened in the cockpit of the Boeing 737 minutes before its nose hit the tarmac and the landing gear collapsed, sending it skidding before it came to a halt in a grassy area.

"At this point in the investigation, no mechanical anomalies or malfunctions have been found," the NTSB said. "A preliminary examination of the nose gear indicated that it failed due to stress overload."

The NTSB said that as the plane approached LaGuardia the captain had been monitoring the landing and suddenly took over from the first officer. NTSB officials said the latest information was only "a factual update" and they could not answer further questions as the probe continues.