Edmund McNally phoned his wife Liz twice following the [WTC 2] aircraft impact. Mr McNally said in his second phone call "Liz, this was a terrorist attack. I can hear explosions below me." [NY Times]
Tom Elliott, WTC 2 survivor: They saw only two firemen going up. They told them there had been an explosion near the 60th floor. [csmonitor]
Kim White, WTC 1 survivor: "We got down as far as the 74th floor ... Then there was another explosion, so we left again by the stairwell." [People]
Explosions occurred below the aircraft impact levels in both towers prior to the collapses.