The director of the U.S. Secret Service is leaving his position, NBC News confirmed on Monday.

USSS Director Randolph Alles, a retired Marine Corps major general who was appointed two years ago, is on his way out the door just the day after the head of the Department of Homeland Security, Kirstjen Nielsen, resigned under pressure on Sunday night. Alles reported to her.

White House press secretary Sarah Sanders said in a statement that President Donald Trump "has selected James M. Murray, a career member of the USSS, to take over as director beginning in May."

Alles was tapped by Trump in April 2017 to lead the agency and was the first director in at least 100 years not to come from the agency's ranks. David Goldman / AP file

More ousters of agency heads within DHS are possible and a number of key positions remains vacant.

An administration official said the decision to seek the resignation of Alles was made 10 to 14 days ago — before a March 28 incident in which a Chinese woman was arrested at Mar-a-Lago without an invitation, touching off security concerns. The woman, Yujing Zhang, was carrying a thumb drive containing malicious software.

"This was not based on any single event," the official said, adding that Alles was told Trump was preparing to make changes in leadership throughout DHS, of which the Secret Service is one component.

In a statement to the agency's workforce on Monday, Alles denied being fired and said he was informed "weeks ago" that there would be across-the-board changes in leadership at DHS.

“No doubt you have seen media reports regarding my 'firing.' I assure you that this is not the case, and in fact was told weeks ago by the Administration that transitions in leadership should be expected across the Department of Homeland Security," Alles said in a statement. "The President has directed an orderly transition in leadership for this agency and I intend to abide by that direction. It is my sincere regret that I was not able to address the workforce prior to this announcement."

He added, "It has been my great honor to serve as Director of the U.S. Secret Service. I want to personally thank you all for a job well done. Your pride, strength and resilience is what has, and will continue to, allow this agency to accomplish great things. Your dedication and sacrifice continue to make the U.S. Secret Service an elite law enforcement agency; one that will remain so well into the future."

Alles also offered praise to Murray, his successor, calling him "a true leader."

CNN first reported Alles' departure.

Senate Minority Leader Chuck Schumer, D-N.Y., called on Alles to testify before Congress about the Mar-a-Lago incident in a statement on Monday

“The outgoing Secret Service director must testify before Congress as soon as possible about the potential security vulnerabilities at Mar-a-Lago involving a Chinese national arrested with malware, and other counterintelligence and national security threats," Schumer said.

"The public and Congress need to know the extent to which adversarial governments — like China — and their agents are attempting to gain access to, or conduct electronic surveillance on, conversations or other information regarding national security at President Trump’s properties," he added.

Alles was tapped by Trump in April 2017 to lead the agency and was the first director in at least 100 years not to come from the agency's ranks.

Alles, who previously served as the acting deputy commissioner of Customs and Border Protection, took the helm of the Secret Service as it faced budget problems amid reports the first family was straining protective resources.

He told USA Today in a 2017 interview that the agency's funding problems were partly caused by the president's large family and their multiple, oft-visited Trump properties both in the U.S. and overseas. "The president has a large family, and our responsibility is required in law,'' Alles told the newspaper. "I can't change that. I have no flexibility.''

Shortly before Alles became the agency's head, a 26-year-old California man wearing a backpack managed to scale the fence along the southern side of the White House — the first such breach reported during Trump's presidency.

Meanwhile, Trump may remove or reassign Claire Grady from her current role as acting Deputy Secretary of Homeland Security, in order to install Kevin McAleenan, the current commissioner of U.S. Customs and Border Protection, as the Acting Secretary for DHS, according to three administration officials.

Her departure may be necessary to avoid any legal challenges to McAleenan’s installation as acting secretary, according to the officials.

As the third-ranking officials in the department, Grady is in line to assume the role as acting secretary Nielsen's resignation. But Trump announced on Sunday that he planned to make McAleenan the acting secretary.

Senate Homeland Security Committee Chairman Ron Johnson, R-Wisc., appeared to reference the recent departures in a statement Monday.

"We are dealing with a humanitarian and security crisis at the border because Congress has failed to act," Johnson said. "In addition to Congressional dysfunction, I am concerned with a growing leadership void within the department tasked with addressing some of the most significant problems facing the nation."