The child was located about half-hour later, asleep in the basement of his own home roughly the same time that the dog, a 110-pound Weimaraner named Geist, confronted an officer, who had entered the backyard of a home at 2400 S. 1500 East when no one answered the door.
Heiden said the "very unfortunate" incident had deeply affected officers, many of whom have dogs of their own.
The officer had entered the backyard to see if the boy may have wandered there. The dog, loose in the fenced backyard, "approached [him] in an aggressive manner. The officer, in turn, shot the dog," Heiden said.
Two shots were fired in all, instantly killing the animal.
The officer was not injured and remained on duty, though Heiden said there was an Internal Affairs Division investigation under way into the shooting.
Heiden said the Utah Humane Society had reached out to the SLCPD to offer special training on dealing with canine aggression. The offer was likely to be accepted, she said.
The department also plans to meet next week with the dog's owner "to see what we can do to help," Heiden said.
The dog's owner was out of town Thursday, burying his pet.
In a written release, the police department said it welcomes "a free and respectful exchange of opinions on this issue" and recommended that comments be sent to firstname.lastname@example.org.