SAN FRANCISCO -- Federal law enforcements officials are monitoring the crash of the Asiana Airlines Boeing 777 on Saturday at San Francisco International Airport, but sources said there was no immediate indication of terrorism or other foul play.

The National Transportation Safety Board is beginning an investigation into the crash. Federal law enforcement sources said the focus now is on a mechanical or operating error.

Another source said 48 passengers received medical care.

The source, who was involved in the rescue effort, said there was no immediate word on the severity of the injuries or whether there were any fatalities.

Some passengers were taken to hospitals.

Tech executive David Eun, who was on the plane, sent out a message on Twitter saying: "Most people are totally calm and trying to let the fire and rescue do their jobs.... Posted updates to let everyone know that majority of passengers seem ok."

A photos taken by a passenger on the flight shows people on the runway and smoke coming from the aircraft. Other photos show the tail of the plane broken off.

In a brief phone interview, a passenger from the flight who didn’t want to give his name told The Times that most passengers were unharmed.

“I just want their families to know,” he said. “Most of the people seem OK and we’re just letting the paramedics do their job.”

San Francisco Fire Department officials told KTVU-TV that injured passengers were taken to the hospital and that there was no immediate word of fatalities. But they stressed the investigation was continuing.

Firefighters put out the fire on the plane, which landed about 11:30 a.m.. The plane was destroyed by the crash and subsequent blaze. San Francisco International has been closed until further notice.

Here are images taken from passengers' social media feed, retweeted by The Times:

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