Pierce County sheriff's Detective Ed Troyer said the children were in the care of their mother's 28-year-old boyfriend while she was away from home for several days for medical treatment.
Troyer says the boyfriend, Stephan Perebeynos, called 911 Thursday afternoon to say he couldn't wake the younger boy. Medics found the child suffering from major head trauma. He's reported in critical condition at a nearby Tacoma hospital.
The boyfriend has been arrested for investigation of assaulting a child.
Troyer said the man claimed the 3-year-old was abducted several days ago but officers don't believe that.
A 7-year-old child was also in the man's care during the mother's absence. That child has been taken into protective custody.
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