The U.S. government is investigating the pilots of the Asiana Airlines passenger jet that crash-landed in San Francisco on Saturday.

National Transportation Safety Board (NTSB) Chairwoman Deborah Hersman said in a news briefing on Monday that the board is carrying out an extensive investigation on Lee Kang-kuk, the pilot who was in the captain's seat, and Lee Jeong-min, the supervisory training captain.

Hersman said the probe is focused on how the pilots flew the Boeing 777, how they were trained, and their flying experience. The transportation agency plans to investigate the pilots for three days. 

The chief investigator confirmed again that while landing, the plane was traveling at speeds well below the target landing speed. 

According to analysis of the plane’s black box, Hersman said the plane was only traveling at a speed of 136 miles per hour, or about 218 kilometers per hour. She said it was much slower than the recommended 157 miles per hour. 

The agency also plans to carry out a probe on the plane’s body. The lower portion of the plane's tail section has been recovered from rocks inside the seawall in front of the runway of San Francisco International Airport.