The hearing came as the U.S. Customs and Border Protection released new figures showing a massive uptick in attempted border crossings in recent months despite the colder weather. Nielsen warned that, at the current pace, the U.S. was on track to encounter close to one million undocumented migrants at the U.S.-Mexico border by the end of the budget year.
"This is not a manufactured crisis," Nielsen told the House Homeland Security Committee. "This is truly an emergency."
Democrats pushed back, pressing her on last year's "zero-tolerance policy" that resulted in nearly 3,000 children being separated from their parents and the conditions in which families are detained.
"No amount of verbal gymnastics will change that she knew the Trump administration was implementing a policy to separate families at the border," the committee's chairman, Democratic Rep. Bernie Thompson, told Neilsen.
Nielsen insisted the administration was merely following the law when it stepped up prosecutions of border crossings, although the U.S. has reverted back to allowing exceptions for most people traveling as families so long as a serious crime hasn't been committed.
Calls for legislative action were echoed by CBP Commissioner Kevin McAleenan in a separate hearing Wednesday. While he and Nielsen continued to demand more border security, McAleenan acknowledged that most of the problems along the border could be fixed with legislative action.
Both officials avoided making a thorough defense of Trump's border wall.
Here are some notable moments from the hearing.
Using cages for kids?Pressed repeatedly by Thompson, the committee chairman, about whether children are still being kept "in cages," Nielsen insisted "they aren't cages" and that the chain-link barriers inside the detention facilities instead helped protect children during processing, citing keeping warring gangs apart as an example.
"I saw the fences that were made as cages and you did, too," Thompson shot back, referring to a tour he took of the detention facility. "All you have to do is admit it. If it's a bad policy, then change it. But don't mislead the committee."
Pregnancy tests for girls aged 10 and upNielsen noted that every girl over the age of 10 is given a pregnancy test and attributed that policy to the dangerous journey migrants face on their way to the southern border.
"As you know, sir, very unfortunately because of the increase in violence, at I.C.E. ... we have to give every girl a pregnancy test over 10," Nielsen said. "This is not a safe journey."
Were parents being deported given an opportunity to take their kids with them?Nielsen was asked whether there had ever been a parent deported under her tenure without being given the opportunity to take their children with them.
"To the best of my knowledge, every parent was afforded that option," she said.
That statement seems to be at odds with what some migrants say has happened. Last week, 24 migrant parents said they were separated and deported without their children.
Families cross the border in record numbersWednesday's hearing came as families and children are crossing the southern border in record numbers, according to U.S. Customs and Border Protection, which released new arrest data Tuesday.
In February, border agents apprehended more than 36,000 families along the southern border, a record high in recent years.
That brings the total number of arrested families over the past five months to more than 130,000. It's an increase of more than 300 percent compared to the same five-month period in 2018.
"It should be very clear from these numbers that we are facing alarming trends in the rising volumes of people illegally crossing our southwest border or arriving at our ports of entry without documents," CBP Commissioner Kevin McAleenan said Tuesday.
McAleenan also acknowledged CBP's enforcement measures are stronger than ever before.
"Our surveillance capabilities, our ability to interdict those who do cross, is at its highest level ever," McAleenan said.
Total border arrests continue to trend upward from recent months with 66,000 reported in February, the highest monthly total since 2010.
ABC's Kendall Karson contributed reporting.