Story highlights Investigation will likely take a year, NTSB says

Two pilots in the cockpit called for the landing to be aborted, NTSB says

No discussion of speed until nine seconds before impact, NTSB says

911 calls pour in to dispatchers from witnesses and passengers alike

An in-depth review of the cockpit voice recorder of Asiana Airlines Flight 214 shows two pilots called for the landing to be aborted before the plane hit a seawall and crashed onto the runway at San Francisco International Airport, the head of the National Transportation Safety Board said Thursday.

The first internal call by one of the three pilots in the cockpit to abort the landing came three seconds before the crash and a second was made by another pilot 1.5 seconds before impact, Deborah Hersman said.

The news came as the NTSB began to wrap up its investigation at the airport, where officials have begun cleaning up the debris left by the crash. Investigators turned over the runway, which had been shuttered since Saturday's crash, late Wednesday, Hersman said.

The investigation is slowly shifting back to NTSB headquarters in Washington, where authorities will work to find a more definitive answer about what led to the crash that killed two 16-year-old Chinese girls and injured more than 180 of the remaining 305 passengers.

JUST WATCHED The final moments of Flight 214 Replay More Videos ... MUST WATCH The final moments of Flight 214 02:00

JUST WATCHED Asiana flight attendants return home Replay More Videos ... MUST WATCH Asiana flight attendants return home 02:08

JUST WATCHED 'We're trying to keep her alive' Replay More Videos ... MUST WATCH 'We're trying to keep her alive' 03:10

JUST WATCHED How do planes work? Replay More Videos ... MUST WATCH How do planes work? 02:59

It will likely take a year before it establishes a cause for the accident, Hersman said. But, she said, it considers the investigation a priority and can make safety recommendations before its final report if necessary.

The NTSB has declined to give a preliminary finding of cause, but investigators have said the Boeing 777 came in too low and too slow.

The cockpit recorder shows there was no discussion of the flight's speed during final approach to the airport until nine seconds before it crash-landed, Hersman said.

A preliminary review of the 777's flight data recorders shows engines and wing flaps responded as they were expected to as the plane made its descent into one of the country's largest airports, Hersman said.

The NTSB chief spent several minutes during a news conference on Thursday clarifying reports that have shown up in South Korean media, including one where the pilot flying the plane reported being temporarily blinded by a flash of light.

Hersman said the pilot at the controls of Flight 214 told investigators that at about 500 feet before crash landing, he briefly saw a bright light "that could have been a reflection of the sun," but he wasn't sure.

The pilot told investigators he did not believe the light affected his ability to fly the plane, as it didn't affect his vision and he could see inside the cockpit, she said.

Additionally, there was no indication that personal electronics, such as mobile phones, aboard the 777 played a role in the accident.

When asked about reports, citing 911 calls from frantic passengers and onlookers, that emergency personnel were slow to respond, Hersman said the investigation was ongoing.

"We are looking at all of that information. We have, really, mountains of information to go through," she said.

Shortly after Asiana Flight 214 crash-landed, passengers and witnesses pleaded with 911 responders to send help -- some frantically, some insistently.

"I'm reporting an airplane crash at SFO (San Francisco International Airport)," an early witness said in calls released by the California Highway Patrol.

"An airplane crash at SFO?" a dispatcher asked.

"Yeah. We were hiking on a trail outside Pacifica, and we heard a giant explosion, and ... an airplane had crashed right there at SFO."

Another caller dialed 911, thinking the response was taking too long.

"We still don't see any firemen or anything," another witness said.

"We are responding, trust me," the operator responded.

Moments before, the 777's main landing gear slammed into a seawall between the airport and San Francisco Bay, spinning the aircraft 360 degrees as it broke into pieces and eventually caught fire.

First responders were on the scene two minutes after the crash to tend to the injured, Hersman said Wednesday. About a minute later, there were firefighters equipped to douse the flames.

Those who could do so poured out of the plane in the aftermath, dialing for help as they escaped.

"We are at the San Francisco airport, and our airplane just crashed upon landing, and we think we need someone here, someone here as soon as possible," a passenger said.

The dispatcher asks: Which runway?

"I don't know what runway. We just literally ran out of the airplane."