The plane was shot down -- U.S. view

A radar system saw a surface-to-air missile system turn on and track an aircraft right before MH17 was shot down, a senior US official tells CNN’s Barbara Starr.

A second system saw a heat signature at the time the airliner was hit, the official explained.

That leads the U.S. to conclude the Malaysian airline was shot down. 

The U.S. is analyzing the trajectory of the missile to try to learn where the attack came from. The U.S. has not determined who was responsible, the source said. 

Other senior US officials tell CNN’s Evan Perez and Elise Labott that the United States so far has “nothing to substantiate” Ukraine’s claims that pro-Russian separatists shot down the plane.