Two autistic Long Island children were able to stroll off their schools’ grounds unnoticed fewer than 48 hours apart last week — and one of them ended up killed while walking along a busy highway.

The terrifying back-to-back security lapses now have Nassau County parents furious and officials scrambling for answers on how the pair of vulnerable kids could have slipped away from supposedly secure campuses in broad daylight. 3

“Our world was shattered,” Danecia Lewis, a grieving sibling of killed 15-year-old Christopher Williams, wrote on the family’s GoFundMe page.

“Instead of ensuring his safety, the school called my mom asking where he was — unaware that he was already gone — a short time later, a detective called her to identify his body,” Lewis said.

Christopher, a student

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