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But she says the children are not being treated inhumanely.
Speaking at a White House briefing Monday, Nielsen says the issue has been growing for years and is the product of loopholes that have created an open border.
Nearly 2,000 minors have been separated from their families in the past six weeks, since Attorney General Jeff Sessions announced a zero tolerance policy where everyone caught crossing the border illegally will be prosecuted. Children can't go to jail with their parents, so they are separated.
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Nielsen says the government has high standards for detention centers and the children are well cared for. She says loopholes in the laws need to be closed so families can stay together.
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AUDIO: Migrant children separated from parents wail at US Customs and Border Protection facility