Musk leads the so-called Department of Government Efficiency, which in the first weeks of Donald Trump's administration has laid off more than 20,000 workers and offered buyouts to another 75,000, across swathes of the government.
Trump administration-appointed officials at the FBI and State Department sent their staff emails telling them not to respond outside their chains of command - a possible sign of tension between members of the Republican administration and the world's richest person in his campaign to slash the 2.3 million-strong federal civilian workforce.
"The FBI, through the office of the director, is in charge of all our review processes," said FBI Director Kash Patel, a Trump appointee, in an email to staff seen by Reuters.
The frantic pace of layoffs has led the federal government in some cases to rehire workers who perform critical functions like securing the nation's nuclear arsenal and dealing with the bird flu outbreak.
Federal workers on Saturday evening received an email instructing them to detail the work they did during the previous week by midnight on Monday and shortly after Musk posted on his X social media site that failing to respond would be taken as a resignation.
Workers at the departments of Defense, Homeland Security, Education and Commerce, as well as at the Federal Deposit Insurance Corporation, the National Institutes of Health and the Internal Revenue Service also were told not to respond pending further guidance, according to sources and emails reviewed by Reuters.
Workers at intelligence agencies likewise will be told not to respond, according to a source.
"To be clear – this is irregular, unexpected, and warrants further validation," wrote a senior executive at the National Centers for Environmental Information, an agency that manages environmental data and is part of the Commerce Department.
Offices within the Justice Department, including the executive office that supports all US attorneys and the department's civil division, told employees not to reply pending additional information.
Employees at the Drug Enforcement Administration, part of the Justice Department, were told to respond, according to an email seen by Reuters.
The American Federation of Government Employees, wrote on social media that it did not believe Musk had the authority to fire employees who did not respond.
The union advised members to ask their supervisors directly whether to reply and to follow their guidance.
The email left some employees even more frustrated and worried after weeks of uncertainty about their futures.
Musk on his social media platform X called the email "a very basic pulse check" and displayed a fictional list he had generated using artificial intelligence.
"EXTREMELY troubling that some parts of government think this is TOO MUCH!! What is wrong with them??" Musk wrote.
In some cases, employees were left unsure how to respond, with some lawyers, expressing concern their work was confidential.
Workers at the Consumer Financial Protection Bureau also received the email, according to people with knowledge of the matter, even though they were all ordered to cease working since early this month, leaving them with little to do.
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