From KGW.com:

“He was working on his rifle,” said Meli.  “He kept pulling the charging handle and hitting the side.”

The break in gunfire allowed Meli to pull out his own gun, but he never took his eyes off the shooter.

From The Truth About Guns:

What we now know was the beginning of gunman Jacob Tyler Roberts’s shooting spree came as a complete shock to everyone. Nick, holding young Noah at the time, turned to Casey and said, “Are you serious?!?”. Casey’s simple answer? “Yes”. As Ashley told us, Nick tossed Noah to her and drew his concealed carry weapon in a single motion.

From The Volokh Conspiracy:

On December 11, Jacob Tyler Roberts killed two people at the mall and wounded a third. Nick Meli, who had a concealed carry license, was apparently at the mall, took out his gun, aimed at Roberts, but didn’t shoot because there was someone running across the field of fire. Meli then took cover, and Roberts shot and killed himself.