Staff with the U.S. Agency for International Development were told overnight that the agency’s Washington headquarters will be closed Monday, according to an email obtained by ABC News from multiple sources.

"Agency personnel normally assigned to work at USAID headquarters will work remote tomorrow, with the exception of personnel with essential on-site and building maintenance functions individually contacted by senior leadership," the email said. "Further guidance will be forthcoming."

The email arrived as Elon Musk, the leader of the Department of Government Efficiency, or DOGE, said he was "in the process" of "shutting down" the agency. Musk claimed he had briefed President Donald Trump on his efforts, which he said Trump supports.

A senior USAID official called Musk's targeting the agency a "mafia-like takeover."

Trump told reporters on Monday that "Elon can't do and won't do anything without our approval. And we'll give him the approval where appropriate."

The president also said he doesn't believe he needs congressional action to shut down the agency.

"I don't think so. If not, when it comes to fraud, if there's fraud, these people are lunatics. And if -- if it comes to fraud, you wouldn't have an act of Congress. And I'm not sure that you would anyway. But we just wanted to do the right thing. That's, that's something that should have been done a long time ago. It went crazy during the Biden administration. They went totally crazy," Trump said.

Secretary of State Marco Rubio has been named acting administrator of the agency, according to a White House memo, and he has notified Congress that a "review of USAID’s foreign assistance activities is underway with an eye towards potential reorganization."

In a letter to key congressional committees obtained by ABC News, Rubio claimed he had the “authority” to make changes to USAID.

“Current foreign assistance processes are severely inefficient and do not substantially benefit the American people. USAID has numerous conflicting, overlapping, and duplicative functions that it shares with the Department of State. Additionally, USAID's systems and processes are not well synthesized, integrated, or coordinated, and often result in discord in the foreign policy and foreign relations of the United States. This undermines the President's ability to carry out foreign relations,” Rubio wrote.

Rubio said he has authorized Peter Marocco, a political appointee who worked at USAID, State and the Commerce Department during Trump's first administration, to serve as deputy administrator and “review” the agency and manage any potential changes.

Rubio said the review could include suspending or eliminating projects or activities and closing and downsizing bureaus and offices. He also wrote that the review could result in “reducing” the size of the workforce and “contracting out or privatizing” work done by federal employees.

Rubio told reporters on Monday in El Salvador during a Latin America trip that the agency is "not functioning" and that it needs to be aligned with U.S. national interests.

"They're not a global charity, these are taxpayer dollars," he said. "People are asking simple questions. What are they doing with the money? We are spending taxpayers' money. We owe the taxpayers assurances that it furthers our national interest."

The developments added to the ongoing turmoil inside USAID, where sources told ABC News that DOGE staffers have moved to take over offices, escalating tensions as more senior staff members are locked out of internal systems, additional employees are placed on administrative leave and the agency’s newly appointed chief of staff resigned.

The Musk-led agency is working to assert control over the USAID, which oversees foreign aid, disaster relief and international development programs.

On Friday night, a dramatic scene played out when a group of individuals who identified themselves as State Department employees and DOGE representatives arrived at the USAID offices in the Ronald Reagan Building and demanded immediate access to every office, according to sources familiar with the incident. It is unclear what legal authority, if any, DOGE representatives have to make such demands.

“It looked like people who dressed up for their first job interview,” a source said.

After initially being denied entry, the group told security guards that if they were not granted entry, they would call the U.S. Marshals Service, sources said. The guards ultimately complied, the source said.

Two top security officials at USAID were placed on administrative leave after they refused to comply on Saturday amid the ongoing turmoil at the agency, sources told ABC News.

Sources told ABC News that more USAID employees were also placed on administration leave, and many were locked out of internal systems amid clashes with DOGE representatives working to take over the agency.

Amid the upheaval, USAID Chief of Staff Matt Hopson resigned just days after Trump appointed him to the position, sources said. It was not immediately clear if Hopson's resignation was due to changes inside the agency.

However, Musk has been joining USAID staff calls during the week amid the shakeup at the agency, according to a source familiar with the matter. And as reports emerged over clashes inside USAID, Musk unloaded a barrage of attacks against the agency across his social media platform, X.

"USAID is a criminal organization. Time for it to die," Musk wrote in one post on Sunday without citing any evidence.

When shown Musk’s posts on X targeting USAID, a senior official for the agency told ABC News, “The warp-speed of this mafia-like takeover has shaken USAID staff to the core.”

“We are not criminals. Where are Republican Senators like Senator Graham and Risch who have supported the good work of this agency in the past? Will they speak out?" the official asked. "Yes, USAID needs reform, like all agencies, but we’d expect this to come with a degree of thought and involvement from Congress.”

Inside the Ronald Reagan Building, posters and flags featuring USAID were taken down and stacked in hallways, sources who were in the building told ABC News. Some USAID employees who were placed on administrative leave had their ID badges and work computers seized, the sources said.

In response to reports on DOGE's activity at USAID, DOGE spokesperson Katie Miller wrote on X, "No classified material was accessed without proper security clearances." The USAID press office did not respond when asked for comment.

The group gained access to USAID’s internal systems on Friday, including the agency’s website and several critical databases, according to sources familiar with the matter. Among the systems was the Development Evaluation Clearinghouse, which houses reports on past and ongoing USAID programs, as well as the Development Information Solution (DIS) -- a system used to track congressionally mandated and performance-related data for all USAID programs worldwide.

Sources also said that the group also seized control of a software system called Phoenix, a program for USAID's financial management system used to track and manage the agency’s budgeting, accounting and financial transactions. The system was down over the weekend, sources said, which has sent shockwaves across contractors for USAID who are fearful they won't be paid for their work. Major firms that manage global supply chains, including those for initiatives like antiretroviral (ARV) drugs and other essential medicines to combat HIV/AIDS, rely on Phoenix.

Sources told ABC News the Trump administration’s foreign aid cuts are being overseen by Peter Marocco, a campaign ally in the Office of Foreign Assistance who was reportedly caught on camera inside the Capitol on Jan. 6, 2021.

Sources also told ABC News that Trump ally Mark Kevin Lloyd has been placed as acting assistant administrator for the Bureau of Conflict Prevention and Stabilization (CPS) at USAID, which oversees a considerable budget. Lloyd, a Tea Party activist, was Trump's Virginia field director in 2016 and later appointed USAID religious freedom adviser in 2020.

The chaos inside the agency has led those familiar with USAID to question whether the overhaul of the agency marks the end of U.S. international development efforts or a strategic consolidation of resources under the State Department. Some see the potential for the administration to leverage USAID’s existing infrastructure -- its contracting officers, global footprint and established contract mechanisms -- to rapidly redirect aid programs in a way that far exceeds the capacity of the State Department.

After the USAID website went dark on Saturday, the agency’s chief information officer sent an internal email stating that the website is “currently unavailable” and that they have no information on when it will be restored, according to an email obtained by ABC News.

“M/CIO is informing users that USAID.gov is currently unavailable. We will send a follow-up notice once there is more information. At this time there is no update on when the site will be available,” the email, which was sent Saturday evening, reads.

A source told ABC News that these notices would typically detail that the team is working to restore access, but that was missing from this notice.

Some staff are taking the "no update" on when the site will be brought back as “a decision made to turn out the (internet) lights.”