Two young siblings went missing Sunday in Jacksonville, Florida, touching off an intense search throughout the night and into Monday afternoon, officials said.

Braxton Williams, 6, and Bri’ya Williams, 5, were last seen playing in front of their residence at the Paradise Village mobile home park, at 10201 W. Beaver St., at around 11:30 a.m., the Jacksonville Sheriff's Office said.

Braxton and Bri'ya Williams. Jacksonivlle Sheriff's Office

The boy is "on the Autism Spectrum but will communicate with others," according to a sheriff's statement.

An Amber Alert for these two missing children was still active on Monday afternoon, authorities said. The search was concentrated on the trailer park and a heavily wooded area north of the park, officials said.

"We're in about the 24-hour mark right now so we're asking the community ... to remain vigilant," Jacksonville Sheriff Mike Williams told reporters on Monday.

"If you live in the area, please search your property including under mobile homes, in sheds, in and around abandoned cars. "

Williams said investigators still don't know if the children wandered off on their own, or were taken away by adults.

"So right now everything is an option. We're working multiple scenarios," Williams said. "Foul play could be a factor, obviously. We just don't know."

AMBER Alert Update:



The investigation has revealed that the clothing reported to be worn by Braxton was not correct, as the red sweater was located inside the residence.



Please continue to RT their photos.#FindBraxtonAndBriya pic.twitter.com/GfK8Lwuppt — Jax Sheriff's Office (@JSOPIO) December 16, 2019

On Sunday night, sheriffs officials said they feared the kids could already be far away from Jacksonville.

"They could realistically be anywhere in Florida if they're not alone," sheriff's spokesman Christian Hancock told reporters on Sunday night.

"If they are alone, obviously, they're in danger because they're 5- and 6-years-old. So we are obviously ramping up the search. We're asking for everyone's assistance in looking."

Authorities are using dogs, dive teams and drones to scour Jacksonville for the kids.

"We're not going to spare any expense. The search will continue throughout the night ... until we find these children," Hancock said.

"We're searching ponds, we're searching trash cans, we're searching everything."