An aviation safety official says accident investigators are looking into what role the shutdown of a key navigational aid may have played in the San Francisco plane crash.

National Transportation Safety Board Chairman Deborah Hersman said the glide slope — a ground-based aid that helps pilots stay on course while landing — had been shut down since June.

She says pilots were sent a notice warning that the glide slope wasn’t available.

Hersman tells CBS’ “Face the Nation” that there were many other navigation tools available to help pilots land. She says investigators will be “taking a look at it all.”

The Asiana Airlines Boeing 777 slammed into the runway on Saturday morning, breaking off its tail and catching fire before slumping to a stop that allowed the lucky ones to flee down emergency slides into thick smoke and a trail of debris. Firefighters doused the flames that burned through the fuselage with foam and water, and police officers on the ground threw utility knives up to crew members so they could cut the seat belts of those who remained trapped as rescue crews removed the injured.

By the time the 307 people on the flight all were accounted for several hours later, two Chinese teenage girls found outside wreckage had been confirmed dead and 182 transported to area hospitals. But as harrowing as the crash was, survivors and witnesses were just as stunned to learn that the toll of deaths and serious injuries wasn’t much higher.

“When you heard that explosion, that loud boom and you saw the black smoke…you just thought, my god, everybody in there is gone,” said Ki Siadatan, who lives a few miles away from San Francisco International Airport and watched the plane’s “wobbly” and “a little bit out of control” approach from his balcony. “My initial reaction was I don’t see how anyone could have made it.”

Vedpal Singh, who was sitting in the middle of the aircraft and survived the crash with his family, said there was no forewarning from the pilot or any crew members before the plane touched down hard and he heard a loud sound.

“We knew something was horrible wrong,” said Singh, who suffered a fractured collarbone and had his arm was in a sling.

“It’s miraculous we survived,” he said.

A visibly shaken Singh said the plane went silent before people tried to get out anyway they could. His 15-year-old son said luggage tumbled from the overhead bins. The entire incident lasted about 10 seconds.

Another passenger, Benjamin Levy, 39, said it looked to him that the plane was flying too low and too close to the bay as it approached the runway. Levy, who was sitting in an emergency exit row, said he felt the pilot try to lift the jet up before it crashed, and thinks the maneuver might have saved some lives.

“Everybody was screaming. I was trying to usher them out,” he recalled of the first seconds after the landing. “I said, ‘Stay calm, stop screaming, help each other out, don’t push.'”

San Francisco Fire Department Chief Joanne Hayes-White said the two who died were found on “the exterior” of the plane. “Having surveyed that area, we’re lucky that there hasn’t been a greater loss,” she said.

Airport spokesman Doug Yakel said 49 people were critically injured and 132 had less significant injuries.

The flight originated in Shanghai, China, and stopped over in Seoul, South Korea, before coming to San Francisco, airport officials said. The airline said there were 16 crew members aboard and 291 passengers. South Korea’s Ministry of Land, Infrastructure and Transport said that the plane’s passengers included 141 Chinese, 77 South Koreans, 61 Americans, three Canadians, three from India, one Japanese, one Vietnamese and one from France, while the nationalities of the remaining three haven’t been confirmed. Thirty of the passengers were children.

Chinese state media identified the dead as two 16-year-old girls who were middle school students in China’s eastern Zhejiang province. China Central Television cited a fax from Asiana Airlines to the Jiangshan city government. They were identified as Ye Mengyuan and Wang Linjia.

At least 70 Chinese students and teachers were on the plane heading to summer camps, according to education authorities in China.

Asiana President Yoon Young-doo said at a televised news conference that it will take time to determine the cause of the crash. But when asked about the possibility of engine or mechanical problems, he said he doesn’t believe they could have been the cause. He said the plane was bought in 2006 but didn’t provide further details or elaborate. Asiana officials later said the plane was also built that year.

Yoon also bowed and offered an apology, “I am bowing my head and extending my deep apology” to the passengers, their families and the South Korean people over the crash, he said.

South Korean President Park Geun-hye offered her condolences to the families of passengers and said her government would make all necessary efforts to help handle the aftermath, according to her spokeswoman Kim Haing.

“I offer my deep condolences to the families of the passengers who suffered from the unexpected Asiana plane crash,” South Korean President Park Geun-hye said, according to her spokeswoman Kim Haing. Park said that the South Korean government will make all necessary efforts to help handle the aftermath, according to Kim.

Based on witness accounts in the news and video of the wreckage, Mike Barr, a former military pilot and accident investigator who teaches aviation safety at the University of Southern California, said it appeared the plane approached the runway too low and something may have caught the runway lip — the seawall at the end of the runway.

San Francisco is one of several airports around the country that border bodies of water that have walls at the end of their runways to prevent planes that overrun a runway from ending up in the water.

Since the plane was about to land, its landing gear would have already been down, Barr said. It’s possible the landing gear or the tail of the plane hit the seawall, he said. If that happened, it would effectively slam the plane into the runway, he said.

Noting that some witnesses reported hearing the plane’s engines rev up just before the crash, Barr said that would be consistent with a pilot who realized at the last minute that the plane was too low and was increasing power to the engines to try to increase altitude. Barr said he could think of no reason why a plane would come in to land that low.

Kate Belding was out jogging just before 11:30 a.m. on a path across the water from the airport when she noticed the plane approaching the runway in a way that “just didn’t look like it was coming in quite right.”

“Then all of a sudden I saw what looked like a cloud of dirt puffing up and then there was a big bang and it kind of looked like the plane maybe bounced (as it neared the ground),” she said. “I couldn’t really tell what happened, but you saw the wings going up and (in) a weird angle.”

Four pilots were aboard the plane and they rotated on a two-person shift during the flight, according to The Ministry of Land, Infrastructure and Transport in South Korea. The two who piloted the plane at the time of crash were Lee Jeong-min and Lee Gang-guk.

Yoon, the Asiana president, described the pilots as “skilled,” saying three had logged more than 10,000 hours each of flight time. He said the fourth had put in almost that much time, but officials later corrected that to say the fourth had logged nearly 5,000 hours. All four are South Koreans.

Asiana is a South Korean airline, second in size to national carrier Korean Air. It has recently tried to expand its presence in the United States, and joined the Star Alliance, which is anchored in the U.S. by United Airlines.

The 777-200 is a long-range plane from Boeing. The twin-engine aircraft is often used for flights from one continent to another because it can travel 12 hours or more without refueling.

The most notable accident involving a 777 occurred on Jan. 17, 2008 at Heathrow Airport in London. British Airways Flight 28 landed hard about 1,000 feet short of the runway and slid onto the start of the runway. The impact broke the 777-200’s landing gear. There were 47 injuries, but no fatalities.

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Lowy reported from Washington, D.C. Associated Press writers Jason Dearen and Sudhin Thanawala in San Francisco, Scott Mayerowitz in New York, Pauline Arrillaga in Phoenix, Hyung-jin Kim in Seoul, South Korea, and Louise Watt in Beijing contributed to this report.