The nation’s top immigration enforcement official confirmed on Monday that a Trump administration policy change is resulting in the separation of undocumented immigrant families, even as she insisted that no such policy exists.

“What has changed is that we no longer exempting entire classes of people who break the law,” declared Homeland Security secretary Kirstjen Nielsen at a press briefing on Monday. “Everyone is subject to prosecution,” and “parents who entered illegally are by definition criminals.”

It’s that change—from a policy that brings civil proceedings against undocumented immigrants to one that levies criminal charges against all adults who illegally enter the U.S.—that has resulted in the highly controversial separation of families at the southern border, where children are being kept in detention facilities as their families are charged and deported.

“This administration did not create a policy of separating families at the border,” Nielsen said.

But the Trump administration’s decision to prosecute every undocumented immigrant is a new approach to the problem, enacted in April 2018. Some high-level White House officials, including chief of staff John Kelly and policy adviser Stephen Miller, have publicly described it as a way to discourage people from attempting to enter the United States illegally.

Nielsen, even after describing the policy change that has thrust the issue into headlines this month, insisted that the administration was simply following the law as written. “It’s not a policy,” she declared, but just the faithful execution of the law as written.

Nielsen echoed President Donald Trump in calling for Congress to reform immigration laws in order to resolve the issue. Trump “wants to actually fix the problem. He wants to secure our border,” said White House press secretary Sarah Huckabee Sanders in remarks following Nielsen’s.

In Congress, political pressure to modify the administration’s policy has built since last week, with even a number of prominent Republican legislators publicly criticizing that approach, including Senate Majority Whip John Cornyn and Sen. Orrin Hatch. “President Trump could stop this policy with a phone call,” Sen. Lindsey Graham said on Friday.

Minutes after Nielsen concluded her remarks, Sen. Ted Cruz announced legislation to halt the separation of families at the border.

Nielsen’s contentious press briefing came shortly after ProPublica released audio from one child detention facility, in which young children can be heard crying for their parents, and one U.S. law enforcement officer jokes about the “orchestra” of cries.

After promising the children were being well cared for, Nielsen was pressed by NBC’s Kristen Welker as to where the girls and toddlers who were taken from their parents where being held. The Department of Health and Human Services has released pictures of boys who are being held in cages inside detention centers, but no public photos have been released of facilities where girls who have been taken from their parents are being held.

After first telling reporters that she had not seen photos of children in cages, Nielsen told Welker that her questions would be better directed to HHS.

Pressed again, she said. “I will look into that, I was not aware that there was another picture.”

Nielsen declined to comment on specific reports about the conditions in which DHS is keeping immigrant children.

"I'm not in a position to deal with hearsay stories,” she said.