The suspect, Kenneth DeHart, 42, who allegedly drove off after the shooting in Maryville, is believed to be armed and dangerous, authorities said.
Gregory Yetman is in police custody following a two-day search.
A witness told Fox News she saw an agitated Laundrie arguing with a server about money and said Petito apologized for his behavior.
The inmates disarmed and fatally shot two officers, police said.
Four women were killed in the attack, and a man who was taken to a local hospital with serious injuries died overnight.
"He's a coward."