Unlike former Department of Homeland Security secretaries John Kelly and Kristjen Nielsen, former Acting Secretary of Homeland Security Kevin McAleenan is a career member of the United States’ vast immigration bureaucracy.

Prior to serving as the head of DHS, McAleenan spent over a decade working for the Customs and Border Protection (CBP) until 2014, when Barack Obama appointed him to be the deputy commissioner of CBP.

“McAleenan is an expert on border protection and immigration policy, but he is ostensibly an Obama guy,” Ali Noorani, executive director of the National Immigration Forum, said during an interview with Boston Public Radio on Monday.

President Donald Trump announced Friday that McAleenan was stepping down.

In April, after the ouster of Nielsen, McAleenan, who had been appointed by President Donald Trump to be the acting head of CBP, became the acting secretary of DHS. At the time, McAleenan struck a different tone from Kelly and Nielsen. As a career member of CBP, he had more insight into the immigration system than his predecessors, and he had publicly criticized the administration's decision to cut aid to Central American nations.

Noorani, however, said that while McAleenan may not have been as outspoken about Trump’s views on immigration, he was instrumental in crafting some of the administration’s harshest policies such as the deportation of asylum seekers to Guatemala and thousands more to Mexico.

“At the end of the day, I don’t think he just implemented the Trump-Miller immigration agenda, he was part of the team who really understood what are the levers within the policy infrastructure to make these changes,” Noorani said. “President Trump and Stephen Miller pretty much got everything they wanted with the smile of Kevin McAleenan.”