An 8-month-old baby boy who drowned in a San Gabriel Valley home had been left unattended in a bathtub with young siblings, according to Los Angeles Sheriff's Department homicide investigators.

Authorities said the infant's mother, who was not identified, was bathing him along with his 1-year-old sister and 2-year-old brother Friday morning when she stepped away for 10 to 15 minutes. When she returned, the baby was face-down in the water and not breathing, Sheriff's Deputy Irys Alvarez said Saturday.

Emergency responders were called to the home in the 1500 block of Pass and Covina Road in Valinda around 8:50 a.m. Friday, Alvarez said. The infant was taken to a hospital, where he was pronounced dead.

The two other children were taken into protective custody by the Los Angeles County Department of Children and Family Services pending a homicide investigation.

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catherine.saillant@latimes.com