President Trump has been trying to reinstate large-scale family separation at the border for months, according to three U.S. officials cited in a Monday report from NBC News. The sources said that Trump has been attempting to restore the highly controversial policy since immigration numbers started rising in January, and that he believes it’s the most effective way to deter migrant families from crossing the U.S.-Mexico border. The position put him at odds with departing Homeland Security Secretary Kirstjen Nielsen, who reportedly told the president that DHS couldn’t legally reinstate the policy and that he would be backtracking from his own June executive order that ended it. It’s not yet clear if the reported dispute was the reason for Nielsen’s abrupt departure.