A Florida sheriff said Monday that foul play hasn't been ruled out in the disappearance of a young girl and her brother.

An Amber Alert was issued Sunday afternoon after the girl, 5-year-old Bri’ya Williams, and the boy, 6-year-old Braxton, were reported missing in Jacksonville.

“We’re working an extensive search operation and we’re also looking at any potential foul play scenarios,” Jacksonville Sheriff Mike Williams told a news conference. “A lot of things are in play right now.”

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Moments earlier he said, “Foul play could be a factor obviously at this point, we just don’t know.” He also told reporters the family was cooperating.

The search was centered on a mobile home park where the children lived.

A massive police presence was reported at the address by Fox 30 Jacksonville.

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“We’re worried about our babies,” Bri’ya and Braxton’s grandmother Roxanne Lloyd told the station. “We want everyone to be praying and if anybody sees them please contact JSO.”

Lloyd told WJXT-TV that Bri'ya and Braxton vanished when their father went inside.

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“My son told me that they was fixing to get ready to have a barbecue and the kids was right outside the door, which is probably only steps away from the kitchen. You know, he could look out the kitchen window and see the kids, and he said that he went in the house briefly to get the meat, and when he came back out, they was gone just that quick.”

She said the children had never wandered away before.