WASHINGTON (AP) - President Barack Obama is expressing his gratitude to the first responders at the scene of the airliner crash in San Francisco. An Asiana Airlines flight from South Korea crashed while landing at San Francisco International Airport on Saturday. Authorities are trying to determine how many have died and how many have been injured in the crash. The White House says in a statement that Obama has directed his team to stay in constant contact with federal, state and local partners as they investigate and respond to the accident. The White House says the president's thoughts and prayers go out to the families of those affected by the crash.