WASHINGTON (AP) _ The acting deputy secretary of Homeland Security was forced to resign Tuesday amid a staff shake-up spurred by President Donald Trump’s growing frustration over the number of Central American migrants crossing the southern border, officials said.

Claire Grady was technically the next in line to replace Kirstjen Nielsen, who resigned Sunday. But Trump chose Kevin McAleenan, the head of U.S. Customs and Border Protection, as acting secretary. That meant Grady had to resign or be fired.

Two officials with direct knowledge of the decision say Grady was forced to resign. The officials were not authorized to speak publicly and spoke to the Associated Press on condition of anonymity.

Grady is a longtime civil servant with more than 28 years of experience at the Departments of Defense and Homeland Security.

With talk that more top officials were likely to be ousted, Republicans expressed public and private concerns about the shake-up orchestrated by the White House and cautioned that leadership changes wouldn’t necessarily solve the problem.

Rep. Elijah Cummings (D., Md.), chairman of the House Oversight Committee, said he may want Nielsen to testify before his panel about Trump’s shake-up of the agency.

“I don’t know why the president has done what he has done, and I certainly would be interested in talking to” officials at the agency, Cummings said.

Asked Tuesday if he will call Nielsen in to talk to the panel, he answered: "We may. But I haven't decided."

Nielsen resigned Sunday after a difficult tenure in which she clashed often with Trump, who wanted a harder-line immigration policy.