A 3-year-old boy was killed and his 2-year-old sister was injured after they crawled away from home and were struck by a train in Indiana, officials said.

Relatives said that Caleb Wilson and sister Ellie Wilson sneaked out from their home in Portage Tuesday morning through a hole in a screen door, the Times of Northwest Indiana reported.

Their mom, Kelsey Benson, was doing laundry when the children made their escape through the damaged door, according to news station WGN.

Bryon Benson, their grandfather, said the family panicked when they realized the kids were missing and quickly set out to look for them.

“By the time the mom went to check the laundry, she came back and said, ‘Dad, the kids are gone,'” Benson told the news station. “I got on my shoes. I went one way, my brother went the other way, she went another way.”

The family, however, didn’t think to check the train tracks four houses down because the children never played there, according to the grandfather.

The freight train conductor told officials that he saw the children but was unable to stop in time to avoid striking them.

Police said the boy was killed on impact and his sister was left with severe head trauma. She was airlifted in critical condition to University of Chicago Comer Children’s Hospital in Chicago.

“It’s dumbfounding to me how they even got out. With three grown adults, not on drugs not drinking state of mind,” Byron Benson told WGN. “Kelsey’s a good mom. She has been from day one.”