The Asiana pilot had four pilots on board when it crashed because it was attempting to land at the conclusion of a 10-hour flight from Seoul, Korea, to San Francisco. Most long international flights take place with two sets of flight crews on board so that pilots can rest when they are not on duty.
Hersman said on Wednesday that she did not suspect anything was amiss with the Asiana pilots, who all survived the crash. However, she said it was normal in U.S. accident investigations to test all pilots for illegal substances.
"I don't have any reason to believe that anything is being withheld," she said. "We've had great cooperation. I know that those pilots were involved in the evacuation and the rescue efforts. They stayed here at the airport for many hours after the accident. I think it's just a question of what the rules and what the policies and procedures and the expectations are."
The NTSB has hinted that pilot error was a factor in the Asiana crash. The agency has said thus far that the Asiana airplane was flying too slowly as it attempted to land at San Francisco International Airport.
Investigators have also revealed that the pilots of the airplane discussed aborting the attempted landing seconds before the plane crashed into the ground.
Hersman said on Wednesday however that the NTSB has not finalized any verdicts about the Asiana accident.
"At this point in the investigation, we are not reaching any conclusions," she said. "We're gathering factual information. We know a lot and what we need to do is correlate all that information. We need to put it together and see what it tells us."
The union for pilots, the Air Line Pilots Association (ALPA), has criticized the NTSB for releasing information about the pilots of the Asiana plane "prematurely."
But Hersman said on Wednesday the agency was just trying to get information out to the public.
"When we're here, we're really here to gather the facts," she said. "We'll be conducting analysis and determining probable cause at a later point. But we want to get information out to the aviation community and to the traveling public while we're here."
Hersman has been the NTSB's lead investigator on the Asiana crash.