The hearing before the Democrat-controlled House Homeland Security Committee began on Wednesday with Mrs Nielsen saying that "vulnerable populations - especially children - are coming into Department of Homeland Security (DHS) custody sicker than ever before".
While Nielsen appeared before the committee, Rep. Rep. Elissa Slotkin (D-MI) asked the DHS secretary how she responded when she first learned that children were being taken away from their parents at the border.
He said more common is a situation where an illegal immigrant claims to be a parent, but is lying.
Republican Congressman Clay Higgins, said that the number of migrants that came to the US-Mexico border last month - 75,000 people - is the same as how many US soldiers landed on French beaches to liberate the country from Nazi Germany.
The Homeland Security secretary said that she would provide the number of children at a later date.
Morning Joe's Mika Brzezinski launched a blistering attack on Homeland Security Sec.
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Nielsen is likely to highlight data that Customs and Border Protection released Tuesday, which showed that more than 76,000 migrant families crossed the southwestern border without authorization in February.
"Would you like me to answer any of your questions, because I would be happy to do so", Nielsen said after repeated interruptions.
"No amount of verbal gymnastics will change that she knew the Trump administration was implementing a policy to separate families at the border", Thompson said.
Democrats questioned Nielsen over the Trump administration's "zero tolerance" prosecution policy, which previous year led to the separation of thousands of migrant children from their parents and generated a fierce backlash, as well as the administration's new restrictions on people seeking asylum at the border. The president grew repeatedly angry with Nielsen for telling him that his ideas to stop immigration would be illegal or improper. And she was asked about conversations with Trump as he declared a national emergency at the border to try to gain funding for his proposed wall between the US and Mexico. During the government's 35-day shutdown over funding for the president's border wall, Nielsen was a fierce advocate for the wall.
Kirstjen Nielsen, secretary of Homeland Security, testifies before the House Homeland Security Committee at the Cannon House Office Building on Wednesday. Homeland Security Committee Chairman Bennie Thompson (D-Miss.) called Trump's declaration "an emergency that does not exist".