Jacob Bisbee was inside the house with his grandmother, uncle and his 4-year-old brother when he walked into the garage where the three dogs were located, said Concord police Lt. Jim Lardieri.
The three dogs attacked the boy and caused severe injuries. He was transported to a local hospital where he was pronounced dead.
The family had five pet pit bulls living in their home on the 1700 block of Trail Creek Court. The remaining two dogs were in the back yard at the time of the attack.
Neighbors do not know much about the family who lives in the house at the end of the cul de sac, but they did notice the pit bulls.
Linda Richardson even called Contra Costa County Animal Services two months ago because a pit bull was tied up in the front yard and barking aggressively. She would not let her granddaughters play outside because she was scared.
"The ropes would come undone or that they would chew through the ropes and run through the neighborhood," Richardson said.
Another neighbor says she was washing her car once when one of the pit bulls ran up to her.
"I was scared and he was barking and I told him 'Go home, go home, and he went like this way,'" said Elizabeth Mazan pointing down the street.
All five dogs were euthanized before police even finished searching for clues at the crime scene.
"They were not requesting the animals back and it's basically a public safety issue, the dog's aggressive behavior," Lt. Joe De Costa said.
Police arrested the child's step-grandfather Thursday afternoon. Steven Hayashi, 52, owned all five dogs, but was not home at the time of the attack.
"Based on the total circumstances we came to the conclusion there was probable cause to arrest him," De Costa said.
In addition to the five pit pulls Contra Costa County Animal Services also removed an illegal ferret from the property and police discovered two shotguns in a truck parked in front of the house. They do not believe they are connected to any illegal activity.
Hayashi will be charged with felony child endangerment and possession of a mischievous animal that resulted in death or great bodily injury. He is being held on $120,000 bail.
Police haven't said if any other family members will be charged.
ABC7's Amy Hollyfield contributed to this report