The Government Accountability Office (GAO) on Friday said that the appointments of acting Homeland Security Secretary Chad Wolf and acting Deputy Ken Cuccinelli were made invalidly -- raising questions about the legality of decisions made during their tenures.

“Because the incorrect official assumed the title of Acting Secretary at that time, subsequent amendments to the order of succession made by that official were invalid and officials who assumed their positions under such amendments, including Chad Wolf and Kenneth Cuccinelli, were named by reference to an invalid order of succession,” the report, by Congress' investigative body, concluded.

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The GAO investigated how the Trump administration filled posts at the Department of Homeland Security (DHS) without Senate confirmation.

It found that, when then-DHS Secretary Kirstjen Nielsen resigned in April 2019, the order of succession dictated that it should have been the director of the Cybersecurity and Infrastructure Security Agency (CISA) who took over. Instead, President Trump appointed Kevin McAleenan -- who was then the head of Customs and Border Protection (CBP).

The opinion found that DHS mistakenly referred to regulations that spelled out the process for a disaster or catastrophic emergency, rather than an executive order that governed the typical order of succession.

“Mr. McAleenan would have been the appropriate official had Secretary Nielsen been unavailable to act during a disaster or catastrophic emergency. That was not the case here,” the report said.

Before he resigned, McAleenan revised the process to allow combine the two processes -- allowing Wolf and Cuccinelli to be appointed. But, the GAO found, because McAleenan was invalidly appointed, so were the changes he made to the order of succession.

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“As such, Mr. McAleenan did not have the authority to amend the Secretary’s existing designation. Accordingly, Messrs. Wolf and Cuccinelli were named to their respective positions of Acting Secretary and Senior Official Performing the Duties of Deputy Secretary by reference to an invalid order of succession, it said.

“We wholeheartedly disagree with the GAO’s baseless report and plan to issue a formal response to this shortly," a DHS spokesperson told Fox News.

The opinion is not binding but is likely to raise questions about the legality of decisions made by Wolf and Cuccinelli during their times at head of the department since taking their posts in November last year -- questions likely to be seized on by those who oppose the Trump administration's tough actions on illegal immigration and its crackdown on violence against federal property in Portland.

GAO said it was not reviewing the legality of the actions taken by the officials and is instead referring it to the department's inspector general.

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Under Wolf, the department has taken a number of moves to crack down on illegal immigration at the border and has made a number of agreements with countries south of the border to step the flow of migrants -- including a series of asylum cooperative agreements (ACA) with Northern Triangle countries.

Recently, Wolf’s DHS also ramped up federal presence in Portland and other cities in response to violence aimed at federal property. Federal law enforcement recently agreed with Oregon officials to scale back presence in Portland as long as the federal courthouse officers had been protecting could be kept safe.

Fox News' Jake Gibson contributed to this report.