"A child of God is dead," said the former FBI senior intelligence adviser, speaking on "The Situation Room" with anchor Wolf Blitzer.
"Cannot we acknowledge in this country that we cannot accept this?" he asked, choking back tears.
"I can't do it, Wolf," he added, breaking down. "I'm sorry. I can't do it."
The victims included students and adults, Broward County Sheriff Scott Israel said.
The suspect, 19-year-old former student Nikolas Cruz, is in custody, the sheriff said. Cruz was expelled for unspecified disciplinary reasons, and police are investigating his digital profile, Israel said. So far, what they've found is "very, very disturbing," he said.
"It's a horrific, horrific day," Israel said. "Just pray for this city. Pray for this school, the parents, the folks that lost their lives."
CORRECTION: A previous version of this story characterized Phil Mudd as an ex-FBI agent, he is an ex-FBI official.
CNN's Emanuella Grinberg, Eric Levenson and Evan Perez contributed to this report.