“Your grant’s immediate termination is necessary to safeguard the interests of the federal government, including its fiscal priorities,” the letters said. “The termination of your grant represents an urgent priority for the administration, and due to exceptional circumstances, adherence to the traditional notification process is not possible.”

The letters, more than a half dozen of which were viewed by The New York Times, were on agency letterhead and bore the signature of Michael McDonald, a longtime N.E.H. official who became acting director of the agency last month, after the previous leader, a Biden appointee, was pressed to resign.

A spokesman for the agency did not respond to a request for comment. The White House referred inquiries to a spokesman for the Office of Management and Budget, which declined to comment.

The letters came days after The Times reported that agency employees had been informed by supervisors that the Department of Government Efficiency, Elon Musk’s cost-cutting group, was seeking cuts of as much of 80 percent of the roughly 180-person staff. Employees were also told that all grants approved during the Biden administration that had not been fully paid out would be canceled.

The extent of cuts to the agency’s hundreds of grants for individual organizations and projects, as well as the ultimate fate of the agency, is not clear.

But the blanket cancellation of funding for humanities councils in 56 states and jurisdictions, which collectively receive about $65 million of the agency’s annual budget of roughly $210 million, sent shock waves through state-level funders.

“The loss of N.E.H. funding to humanities councils will decimate the ability of these nonprofits to serve communities in their states, eliminating programs that are essential to each state’s cultural infrastructure,” the Federation of State Humanities Councils said in an “urgent appeal” to members on Thursday morning.

The legal basis for the cancellations remains unclear. Observers have noted that the creation and funding of state humanities councils was specifically mandated in the legislation passed by Congress.

Representative Chellie Pingree, Democrat of Maine and the ranking minority member on the House Appropriations subcommittee that oversees the endowment, said in a statement that the termination of the grants was “devastating and outrageous.”

“Let’s be clear: These grants were already awarded and use funds already appropriated by Congress on a bipartisan basis,” she said. “The notion that these terminations are justified by a sudden shift in ‘federal priorities’ is nonsense. This is ideological targeting — pure and simple. And it is happening with no input from Congress or the public.”

In recent weeks, Department of Government Efficiency employees have made repeated visits to the office. Two were at the agency’s office on Wednesday afternoon and spoke with leadership, according to a person who was present. Leadership of the humanities agency has yet to communicate any information about the cuts to staff members, employees said.

The most recent update to its funding programs was posted on its website on March 20.

The N.E.H. is a small part of the alphabet soup of federal agencies, and is often conflated with the National Endowment for the Arts. But while tiny by federal standards, it is a crucial source of funding for museums, historical sites, scholarship and both school-based and community-based projects across the country.

Since the agency’s creation in 1965, it has provided more than $6.4 billion to support more than 70,000 projects in all 50 states and U.S. jurisdictions, according to its website. Supported projects have included more than 9,000 books (including 20 that went on to win Pulitzer Prizes) and more than 500 film and radio programs, including Ken Burns’s landmark 1990 documentary “The Civil War,” which received about a third of its budget from the agency.

Agency grants have supported archaeological excavations at Jamestown, Va., research on the Dead Sea Scrolls and publication of the full records of the voyage of Lewis and Clark. And the agency has funded numerous multidecade, multivolume projects dedicated to gathering and publishing the papers of figures including 12 presidents, Mark Twain, Thomas Edison, the Rev. Dr. Martin Luther King Jr. and Ernest Hemingway.

Cathy Gorn, the executive director of National History Day, a nonprofit group promoting history education, said the cuts would be especially painful at a time when support for the humanities in schools, universities and elsewhere was already small, and shrinking.

“We are in danger of not being able to help this next generation of American citizens understand the nation’s past,” she said. “There’s a lot to lose here.”

National History Day, Gorn said, had received notices canceling the unpaid remainder of two multiyear grants totaling about $825,000.

The federal endowment is crucial to many small humanities projects nationwide since by law, it funnels a large chunk of its budget directly to state councils, which then fund projects within their states. State support differs by population. In the 2024 fiscal year, Texas received about $3.4 million, while North Dakota was allocated about $900,000.

For many state councils, particularly in smaller states without major cultural organizations and a strong philanthropic base, the federal agency provides all or virtually all funding. Without those grants, some may simply collapse.

Caroline Lowery, the executive director of Oklahoma Humanities, said her organization, which has seven employees, received roughly $1 million in operational support from the agency each year, which amounts to about 75 percent of its budget. That money is then used to support projects serving all of Oklahoma’s 77 counties, most of which, she said, are rural and lack any other humanities infrastructure.

“The impact will be devastating statewide,” she said. “There will be an immediate loss of support for programs that serve veterans, programs that serve rural communities.”

Projects have included an oral history project with survivors of the 1921 Tulsa race massacre, and an effort to digitize news coverage and other records relating to the 1995 bombing of a federal building in Oklahoma City.

“Our history will literally be lost,” she said. “We are the stopgap. We are the institution that is making sure Oklahoma’s stories are preserved.”

Mark Santow, the founder of the Providence Clemente Veterans’ Initiative, a group that provides free cultural and educational programs for veterans in Massachusetts and Rhode Island, said his group had been informed that its current two-year, $99,000 grant, of which about $18,000 had not yet been received, was canceled. Separate grants it receives from Rhode Island’s state council were also in jeopardy.

Upcoming programs, like a field trip to Civil War battlefields, were uncertain, Santow said.

“As always, the vets in our community will quietly bear the burden of these ill-considered decisions,” he said. That leaders have the power to make them, he added, “doesn’t make it right.”