Washington (CNN) Homeland Security Secretary Kirstjen Nielsen and House Democrats sparred Wednesday over the Trump administration's controversial immigration policies that resulted in children being separated from their parents and President Donald Trump's declaration of a national emergency.

Nielsen defended the administration's "zero tolerance" policy, which called for the criminal prosecution of adults crossing the border illegally and, as a result, the separation of families. Democratic lawmakers grilled the secretary over the policy, asking her when she knew about it and whether it was intended to deter migrants from approaching the southern border.

In all instances, Nielsen took a defensive stance, arguing that it was not policy, but law, referring to the criminal prosecutions.

"We enforce the law," she told Rep. Kathleen Rice.

Nielsen has previously testified that the department "never had a policy for family separation."

On Wednesday, she said the policy wasn't intended to be a deterrent to families crossing the border. But in January, Sen. Jeff Merkley, D-Oregon, released a 2017 memo drafted by senior officials in the Department of Homeland Security and the Justice Department detailing the plans.

Under a section dubbed "short term (next 30 days) options," the memo says: "Separate Family Units. Announce that DHS is considering separating family units, placing the adults in adult detention and placing the minors under the age of 18 in the custody of HHS as unaccompanied alien children."

Homeland Security Committee Chairman Bennie Thompson said the memo shows Nielsen was aware of the impact of the zero tolerance policy.

"No amount of verbal gymnastics will change that she knew the Trump administration was implementing policy to separate families at the border," the Mississippi Democrat said.

Trump statements

Lawmakers also pressed Nielsen to respond to Trump's statements on the US-Mexico border.

Thompson asked about the President's statement that he "didn't need to" declare a national emergency declaration. Nielsen defended the move, calling it a "legitimate national emergency" but would not disclose details of her discussions with the President about the issue.

"My conversations, of course, with the President, generally speaking, are protected under privilege, but what I would say is his explanation in general, in public, has been that he hoped Congress would act, that it didn't have to come to issuing an emergency declaration if Congress had met his request to fund the resources that CBP has requested," she said.

She was also asked to respond to Trump's remarks about apprehensions reaching an all-time high. Nielsen carefully responded, withholding direct comment on Trump's statements.

"Is the President misleading the American people? Is he accurate in what he is saying? It's the largest amount of detentions -- apprehensions at the border?" said Rep. Jim Langevin, D-Rhode Island.

"In some categories we have had record-breaking apprehensions," Nielsen said.

Nielsen described an overwhelmed system at the border as a result of a surge of migrants. Nielsen said that the US is on track to apprehend 900,000 undocumented immigrants at the southern border this year.

"The projections are dire," she told the panel.

2006 was the last year to reach over a million apprehensions (1,089,092) and 2007 was the next closest with 876,704.

Increase in families coming to border

On Tuesday, Customs and Border Protection officials announced that families and unaccompanied children make up more than 60% of apprehensions along the southern border. They're predominantly from Guatemala, Honduras and El Salvador. More than 76,000 people were apprehended or deemed inadmissible at a port of entry in February.

This, combined with an influx of groups of over 100 people illegally crossing the border, has presented a unique challenge to CBP. In previous years, the agency dealt primarily with single adults from Mexico who could be quickly returned. But because most crossings now are family members from the so-called Northern Triangle countries, the US does not have the infrastructure to accommodate the influx.

As a result, DHS, along with the President, has dubbed the situation along the southern border a national security and humanitarian crisis. Immigration advocates, for their part, have pushed back on it being a national security crisis.

The administration thus far has relied on deterrence to stem the flow of migrants, through policies like "zero tolerance" and the so-called Migrant Protection Protocols, informally known as "Remain in Mexico."

The US began to implement the policy, which requires some asylum seekers to await their immigration court hearings in Mexico, at the San Ysidro border crossing and plans to expand it to other locations along the border. A coalition of immigrant advocacy groups has asked a federal judge for a restraining order that would block the program.

Similarly, the latest revelations about the administration's "zero tolerance" policy has renewed scrutiny of the administration. Government reports have painted a picture of chaos and confusion since the policy ended last year. In January, a Health and Human Services Department inspector general report that found thousands more children had been separated than previously acknowledged.