The acting head of US Homeland Security has resigned after only six months, becoming the fourth person to leave the role during Donald Trump's presidency.
Kevin McAleenan, 48, was made acting head of Homeland Security after Mr Trump called for Kirstjen Nielsen's resignation.
Under Mr Trump, the position has involved implementing controversial immigration policies, particularly along the US-Mexico border.
In a tweet the US president said that McAleenan was stepping down to spend more time with his family.
However, the pair are believed to have had a difficult relationship, with Mr McAleenan criticising the tone of the immigration debate.
Before becoming acting secretary, Mr McAleenan served as commissioner of US Customs and Border Protection, one of the agencies operating under the Department of Homeland Security (DHS).
"Kevin now, after many years in Government, wants to spend more time with his family and go to the private sector," Mr Trump said on Twitter.
"I will be announcing the new Acting Secretary next week. Many wonderful candidates," he said.
Mr Trump said he and Mr McAleenan had "worked well together with Border Crossings being way down".
Despite the praise, Mr Trump never formally nominated him to run the agency.
In a statement, Mr McAleenan said during his six-month tenure the Department of Homeland Security had "made tremendous progress mitigating the border security and humanitarian crisis we faced this year".
US Customs and Border Protection said this week that arrests at the US-Mexico border fell in September for the fourth month in the row.
It said there were just over 52,000 migrants either apprehended or encountered at the southwest border in September, down almost 65 per cent from a peak in May of 144,000.
The bulk of arriving migrants are from Central America, many of them families, fleeing situations of violence and poverty at home and often seeking asylum in the United States.
