Search teams in western Arkansas were scouring Saturday for any sign of a 9-year-old boy missing for nearly a week -- as investigators found the bodies of at least three relatives killed, including his mother and baby sister.

The missing boy, Reilly Scarbrough, is "in immediate danger" and hasn't been seen since at least Sunday, according to the National Center for Missing & Exploited Children.

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Crews found the body of his mother, 43-year-old Bethany Jo Wester, in a creek Tuesday near the town of Cove in far western Arkansas. On Thursday, they found the body of the boy's great-uncle, 66-year-old Steven Payne, in the nearby town of Hatfield, Fox 16 reported.

On Friday, the search took another grim turn, when the body of 2-year-old Acelynn Wester turned up in the woods not far from where they found the mother's remains, according to Polk County Sheriff Scott Sawyer.

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The deaths are being investigated as homicides, said Sawyer, who choked back tears at a news conference late Friday as he promised to "seek justice for the families" of the victims.

Officers from eight agencies are searching for the boy, Sawyer said, adding that he was keeping up hope that the child was alive.

"Somebody knows where he's at. Someone has information," Sawyer said. "We need to find Reilly. We're going on the premise Reilly is alive."

The sheriff did not say how the three were killed or name any possible suspects or motives in the deaths. The sheriff also didn't provide information on where the children's other relatives, including a father, might be.

"With their mother being found dead and their uncle being found dead, that ups the criticalness of the case," said Rebecca Kovar, the senior program manager at the national nonprofit. "The longer a child is missing, the more critical the situation becomes."

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The Associated Press contributed to this report.