"I feel like, and the evidence is really clear, that this is intentional," Underwood said to audible gasps in the committee room. "It’s a policy choice."
Acting Homeland Security Secretary Kevin McAleenan called her assertion an "appalling accusation" before the hearing took a recess.
Underwood's comments came as McAleenan was testifying before a House committee to defend his department's budget request, but he was repeatedly questioned about the treatment of migrant children along the border.
That's when Underwood, a nurse, began asking McAleenan about his department's treatment of migrant children, pointing to recent photos of migrants sleeping on the ground outside Border Patrol facilities despite increased funding from Congress to provide humanitarian assistance at the southern border.
Five children from Guatemala have died since December amid a record-setting surge of families and children pouring across the border.