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Alicia Ortez. (San Pablo Police Department)

SAN PABLO -- A man was found shot to death here Monday morning, and his three children and their mother are missing, police said.

The man, whose name was not released late Monday, was found dead at the scene from a gunshot wound at 7:55 a.m. in the backyard of a home in the 1600 block of 16th Street, police said.

An Amber Alert has been issued for Hugo Anguiano, 12, Merari Anguiano, 6, and Jenh Carlos Anguiano, 2. The three children lived with the man, and the mother lives separately, police said. It was not known where the children were at the time of the shooting.

Their mother, Alicia Ortez, 35, from the San Pablo-Richmond area, is also missing. Police believe the children are with their mother.

Police have not said if Ortez is a suspect in the man's death and said investigators have not yet determined a motive for the killing.

Anyone with information about the case should call San Pablo police at 510-215-3150 or the West Contra Costa Crime Stoppers tip line at 510-799-8255.

Follow Kristin J. Bender at Twitter.com/kjbender.