SAN FRANCISCO — The head of Asiana Airlines apologized Sunday for the weekend plane crash that killed two teenage passengers.

"I sincerely apologize over the accident, and to the passengers on board and their families," Yoon Young-doo, Asiana's president, told reporters at a televised news conference in Seoul.

He described the pilots involved as "skilled" and said it could take time to determine what went wrong.

Seoul-based Yonhap News Agency said the two victims were born in 1996 and 1997 and were from China. Asiana Flight 214 originated in Shanghai and stopped in Seoul before flying to San Francisco International Airport, where it crashed late Saturday morning.

The bodies of the teens were found on the runway, said San Francisco Fire Chief Joanne Hayes-White. It was not clear whether they had been pulled from the plane or ejected.

Seoul-based Asiana is South Korea's second-largest airline.

National Transportation Safety Board investigators have arrived at the airport to investigate the crash.

Dozens of survivors were taken to hospitals. Passengers said that despite the chaos, most aboard Flight 214, which originated in Shanghai with a stop in Seoul, seemed able to exit quickly and walk from the wreckage without help.

The cause was unclear. Federal investigators were looking into whether the plane clipped a sea wall separating the runway from San Francisco Bay, according to a person involved in the investigation. Officials said there was no indication that terrorism was involved.

"We were too low, too soon," said passenger Benjamin Levy, who described looking out his window, seeing piers in the bay and thinking the piers were closer to the plane than they should have been.

The pilot of the Boeing 777 seemed to rev the engines "just as we were about to hit the water," Levy said. "The pilot must have realized [and] tried to pull the plane back up. ... We hit pretty hard. I thought the wheels were gone for sure."

Levy, a 39-year-old San Francisco technology executive who'd traveled to Asia on a business trip, heard screams as the plane, carrying 291 passengers and 16 crew members, slammed into the ground.

When emergency crews arrived, the white, wide-bodied jet was emitting black and white smoke as it sat on a stretch of brown grass near the tarmac. The tail was gone and pieces of the plane littered the runway. Flames had burned a gaping hole through the top of the aircraft.

Multiple sources said there was no reported trouble or declared emergency on the plane before it landed.

Asked at a news conference if pilot error was a factor, Deborah Hersman, chairwoman of the NTSB, said "everything's on table at this point. We have to gather all the facts before we reach any conclusions."

Hayes-White, the fire chief, said a number of passengers were seen emerging from the waters of San Francisco Bay when first responders arrived on the scene. However, the wreckage was a short distance away and Hayes-White said "the assumption" is that survivors may have immersed in the water themselves to douse flames.

Hayes-White added that when her crews arrived, emergency chutes had already been deployed "and we were observing multiple people coming down the chutes and walking to safety, which was a good thing." San Mateo County firefighters performed search-and-rescue operations inside the aircraft, she added.

On Saturday night, all 307 on board had been accounted for, authorities said. One hundred eighty-two people had been transported to hospitals, including 49 in serious condition. Among the passengers were 77 Korean citizens, 141 Chinese, 61 Americans and one Japanese, according to South Korea-based Asiana.

Flight 214, like all aircraft landing in San Francisco on the sunny clear morning, was using visual flight rules, an airport spokesman said. FBI Special Agent in Charge David Johnson said his agency will be assisting the NTSB to determine the cause of the accident.

Moments after the crash, a United Airlines pilot in another plane announced welcome news to the airport control tower: There were survivors.