A former employee and a 16-year-old boy have been arrested in the shooting death of a pizzeria assistant manager. Sacramento County sheriff's investigators said they believe that Jeffrey Alan Cattera, 22, who was fired about a month ago, was the triggerman in the slaying of Robert Colon, 24. The assailants stole about $3,000. Cattera and the 16-year-old, who was not identified because of his age, were arrested several hours after Colon was shot in the chest in a Round Table Pizza restaurant in suburban Citrus Heights. Colon died one week before he was to be promoted to manager of a new restaurant in Roseville, said restaurant owner Joe Brennan. Two young men wearing ski masks and armed with handguns entered through a rear door, apparently let in by employees who thought a delivery person had arrived, deputies said. One man ordered three employees into a walk-in refrigerator and the other shot Colon in the office, where he was doing some accounting work, officers said.