"It's a terrible tragedy, " neighbor John Carmody told KTVU as he stood across the street from the family's house. "I feel for them."
Police say they were originally told the child was four years old, but clarified later that he was three. They were called to the 100 block of California Street just past 4 p.m. with a frantic 911 call.
"He was playing in the backyard and his parents were inside the house," explained Vallejo Police Lt. Herman Robinson. "At some point, they didn't hear the child playing in the back and when they went to check on him, found him floating in the pool."
Robinson said the pool has a deep end of seven feet, and has no barrier around it.
"You need either a cover on a pool, or some sort of structure, like a fence" he specified.
About 6 p.m., as investigators wrapped up their investigation, friends and relatives could be seen arriving and entering the home with hugs and tears.
The boy's parents had rushed to Kaiser Hospital with the ambulance, but there was nothing to be done. They came home without their son.
"I saw the family enter the home and I heard the cries and the moans from the mother," Carmody told KTVU. "It was pretty terrible to me."
Only nine days ago, a one-year-old girl drowned in a backyard pool in Concord. Her mother was home at the time, but didn't realize the toddler had wandered outside.
In both instances, panicked parents tried to revive their children, then rescue paramedics took over.
In both cases, the kids had been in the water for too long.
"Oh golly. Poor baby. I'm so sorry to hear that," neighbor Russell Turner told KTVU. His house is directly behind the victims' in Vallejo.
Turner says the couple is friendly and has a younger child as well. He's observed them around the pool, and described them as very attentive.
"I know they keep the boy out of the area," recalled Turner. "Whenever he's out there, there's always someone with him."
But apparently not this time.
Turner said the family moved in about a year ago and has been renovating the home. It's possible they were distracted by the work they were doing, as their son played near the water.
"A million pools in America don't have covers, " observed Turner, "It's just one of those sad situations that something happened."