Watch: Second moment of silence in NYC at 9:03 a.m. when hijackers rammed plane into WTC South Tower

Hijackers crashed United Airlines Flight 175 into the South Tower of the World Trade Center at 9:03 a.m. ET on Sept. 11, 2001. The second moment of silence is one of seven to commemorate the events 25 years ago that changed the U.S. forever.