More on the terrorist attack:

Multiple U.S. officials briefed on the investigation told NBC News that preliminary reports indicate that a single explosion took place outside the arena on the southwest side, opposite the Manchester Victoria transit station, which is part of the greater arena complex.

Two US law enforcement sources tell CBS News that the Manchester attack was conducted by a suicide bomber who traveled by tube to Victoria Station and detonated device as people were leaving the concert. Pieces of ball bearings found at scene.