SAN FRANCISCO/WASHINGTON -- Asiana Flight 214 was flying significantly slower than its pilots had intended, and they made a failed attempt to abort landing just 1.5 seconds before smashing into a runway at a San Francisco airport, U.S. investigators said Sunday.

The National Transportation Safety Board (NTSB) announced the preliminary results of its analysis of the so-called black box from the passenger jet, which crash-landed on Saturday.

Two Chinese girls were killed and more than 180 people were injured in the accident.

The Boeing-777's pilots targeted 137 knots, or 254 kilometers per hour, as the plane approached the runway bordering the sea, according to NTSB chief Deborah Hersman.

"We have to take another look at the raw data and corroborate it with radar and air traffic information to make sure we have a very precise speed. But again, we are not talking about a few knots here or there. We're talking about a significant amount of speed below 137," she told reporters.

The pilots had not made any prior distress call to the airport authorities, suggesting there was no abnormality in the engine or other flight systems, she said.

Hersman also stated that South Korean officials have formally joined the probe, which is expected to take 12-18 months.

The South Korean team, composed of six transportation ministry officials and some Asiana staff, arrived in San Francisco earlier in the day to assist the investigation by the U.S. air safety authorities.

"It takes more than one year to look into the cause of this kind of accident," said Park Jung-gwon, a South Korean transportation ministry official heading the team.

"We plan to actively take part in a probe by the U.S. aviation authorities. But the U.S. is in charge of the investigation itself," Park told reporters.

Hours after the accident, U.S. officials brought the doomed plane's black boxes -- a flight data recorder and the cockpit's voice recording device -- to the headquarters of the NTSB in Washington, D.C.

"At this point, everything is still on the table for us... There is a lot of work ahead of us," Hersman reiterated.

Carrying 291 passengers and 16 crew members, the jet smashed into a runway of the San Francisco airport at around 11:30 a.m.

Saturday (local time). The tail broke off and the top of the fuselage was burnt out.

Skies were relatively clear at the time of the incident, with the Federal Bureau of Investigation saying there is no indication of any link to a terrorist attack.

In Seoul, Asiana officials said the seven-year-old plane did not experience any technical glitches.

"For now, we acknowledge that there were no problems caused by the 777-200 plane or engines," Yoon Young doo, the president and CEO of the country's second-largest airliner, said at a press conference.

He also dismissed the possibility of pilot error, saying all of his company's pilots aboard the jet were highly skilled and experienced.

All of them are veterans, Yoon said, with around 10,000 hours of flying experience.

The plane originated from Shanghai, China and stopped over at Incheon International Airport, west of Seoul, South Korea, before heading to San Francisco.

U.S. investigators, meanwhile, noted a key navigational device at the airport was shut down when the plane attempted to land.

The glide slope, which is a ground-based aid that helps pilots stay on course while landing, had been shut down since June at the airport, NTSB chief Hersman said, adding that this might have played a role in the crash.

She stressed, however, that it is premature to connect the shutdown of the equipment with the accident as pilots can use other tools such as GPS to aid in landing.