The Franklin School at 13th and K streets NW. (Matt McClain/The Washington Post)

District officials have ordered construction work to stop on Franklin School days after they toured the national landmark and found that developers had destroyed “significant amounts” of historical material.

The city-owned Franklin School at 13th and K streets NW is being converted into Planet Word, a privately run museum dedicated to linguistics. The 1869 building, a National Historic Landmark, is one of just 18 structures in the city with the highest level of historical preservation protection, and strict rules govern alteration of the building’s interior and exterior.

The D.C. Department of Consumer and Regulatory Affairs issued a stop-work order at the property Tuesday. The order means developers who began work on the building in June cannot proceed until they obtain a new permit. They must also pay a fine, which will total half the cost of the new permit — the price of which will vary depending on its scope.

In an Aug. 30 letter to philanthropist Ann Friedman, who is leading the $50 million redevelopment, the D.C. government’s top preservationist admonished her workers for exceeding the terms of her original permit. A team of local and national preservation officials had toured the building earlier that day.

“We are writing to document that significant amounts of historic fabric have been removed from the building without prior authorization,” wrote David Maloney, the District’s state historic preservation officer at the Planning Department.

The letter, which The Washington Post obtained, said all original plaster wall finishes and ceilings in the building’s former classrooms had been removed. Brick walls, nearly all the wainscoting, baseboards and tin ceilings were also gone, Maloney wrote.

Friedman declined to comment on the stop-work order, but said that the building’s history is part of what drew her to the property and she is honored to be the one to restore it. She’s committed to working with local and federal agencies to preserve that history, she said.

“The Franklin School is an architectural landmark and an iconic artifact of Washington D.C.’s rich history,” Friedman said.

The city has known about the violations since at least Aug. 17, an official said. Preservation advocates are relieved the city has stopped work on the property, but it should have done so sooner, said Rebecca Miller, executive director of the nonprofit D.C. Preservation League.

“DCPL is pleased that the city finally took action to issue a stop-work order on the building,” Miller said. “The city has, however, been aware of the permit violation for several weeks and should have acted more swiftly to ensure the protection of this irreplaceable historic resource.”

Work on the site constitutes a “blatant disregard for the Historic Preservation Act,” Miller said.

In a statement, Brian T. Kenner, the deputy mayor for planning and economic development, said the stop-work order was the result of a “thorough investigation.”

“The Bowser Administration is working with the development team to remediate the issue and put proper controls in place to continue the transformation of Franklin School into a world-class museum in the heart of the District,” said Kenner, whose office is working with Friedman on the redevelopment.

A spokeswoman for Kenner said it took the city time to determine the extent of the damage and decide on the best course of action. While much of the historical material is destroyed or sitting in a landfill, the spokeswoman said the city is determined to prevent further damage.

The Planet Word museum, scheduled to open in late 2019, will be the building’s first occupant since a city-run homeless shelter closed in 2008. In 2011, a group of Occupy D.C. protesters briefly set up camp there before they were arrested.

Built in 1869, the Franklin School was designed by Smithsonian architect Adolf Cluss and became the District’s first high school.

It was the site of the first transmission of a wireless message in 1880, when Alexander Graham Bell used one of his inventions, a photophone, to send a message over a beam of light to a window in a nearby building.