(CNN) -- A disgruntled employee shot to death the owners of a store in Boulder, Colorado, and then may have turned the gun on himself, police said Tuesday.
Police found Sean Griffin, 40, and his wife Staci Griffin, 41, dead in a back room of the Boulder Stove & Flooring on Monday morning. Also dead was Robert Montgomery, 50.
Montgomery had a 9 mm Smith & Wesson handgun in his hand and appeared to have suffered a single gunshot wound, said police spokeswoman Sarah Huntley. The Griffins sustained multiple wounds.
Montgomery told friends and wrote in a journal that he was upset over a recent change in sales commission and bonus policy at work, Huntley said. He purchased the gun on May 7, she said.
Police said Montgomery had been employed at Boulder Stove & Flooring since 2003.
A salesman and a customer were inside the store when the gunshots rang out at 11:09 a.m. They jumped in the customer's vehicle, drove away and called 911, Huntley said.
Police arrived minutes later to find the three dead.
An autopsy will determine whether Montgomery was the shooter, but Huntley said police suspect him after finding the same kind of Cor-Bon 9 mm ammunition at his house that was used in the shooting. Police said they found 13 spent shell casings around the scene of the shootings with one round still in the chamber of the gun. No rounds were left in the magazine, they said. At his home, they found a box of Cor-Bon 9 mm ammunition with 14 rounds missing.
Investigators said they found a second magazine in Montgomery's backpack loaded with 10 rounds of Eagle brand 9 mm ammunition. At his home, they found an empty box of Eagle 9 mm ammunition.