The acting commissioner of the US Customs and Border Protection (CBP) agency, John Sanders, is resigning and will leave his post on July 5, amid an outcry over the treatment of detained migrant children.
His departure comes amid mounting public backlash over alarmingly unsanitary conditions at an overcrowded migrant facility in Clint, Texas where the discovery of poor care and lack of access to showers or clean clothing prompted the relocation of 250 detained children.
The situation has led immigration activists and Democrats to step up criticism of President Donald Trump's hard-line immigration policies.
Mr Sanders, whose resignation was first reported by The New York Times, has led the agency since April, when Mr Trump reshuffled the management of US immigration agencies under the Department of Homeland Security.
Before taking over CBP, he was the agency's chief operating officer and had also been the Transportation Security Administration's chief technology officer.
Mr Trump said he had not ask for Mr Sanders to quit, calling him a good man and saying he did not know why he decided to resign.
Dealing with a surge of migrants at the US -Mexico border has been a priority for Mr Trump - but the president has proven unable to push most of his goals through Congress.
On Tuesday, Democrats in the House of Representatives said they plan to approve $4.5 billion (£3.5 billion) in emergency funding to address the crisis caused by the migrant surge, but the measure has drawn a veto threat from Mr Trump.
"This week we have to solve the humanitarian crisis," Hakeem Jeffries, chair of the House Democratic Caucus Chairman said, predicting that the funding package would pass the House with a "strong Democratic vote."
But lawmakers were also rushing to add language before the vote to mandate better health and nutrition standards at border facilities.
The changes were being made after some liberal Democrats expressed alarm that not enough was being done to improve conditions at the border, where the number of migrants apprehended surged in May to the highest level since 2006.