Repairs made last year to cracks that zigzag across the monument at the Tomb of the Unknowns at Arlington National Cemetery are failing, and officials said Wednesday that they are trying to figure out how to fix them.

Initially, the cemetery was to begin working on the monument this week. But historic preservationists, upset that they were notified of the problems just a week before new repairs were to start, accused the cemetery of moving too fast and possibly performing a shoddy job that could jeopardize the 48-ton white marble monument.

“This is racing forward,” said Robert Nieweg, director of the National Trust for Historic Preservation’s Southern Field Office. “They’ve already failed once. Why would they rush into a second repair when the experts don’t know for certain why the first one failed? . . . If you don’t do it right, you can harm the historic resource.”

But Army officials who oversee the cemetery said they were happy to slow the process so that preservationists can weigh in on the complicated project and be assured that the monument will be treated with the utmost care. They also said that because of the recent hot weather, which would wreak havoc on the grout used to repair the cracks, the work would not have moved forward this week anyway.

“We are committed to being transparent and seeking subject matter expert advice when it comes to things like this,” said Col. Tori Bruzese, the cemetery’s engineer. “We’re not in a rush to failure.”

For years, the Trust has criticized the Army’s stewardship of the nation’s premier military burial ground, saying it hasn’t done enough to preserve its historic monuments.

Earlier this year, it blasted the cemetery when a pair of nine-foot-tall decorative urns that once flanked the stage of the cemetery’s Memorial Amphitheater were put up for sale by an Alexandria auction house. It was not clear how the urns ended up in private hands, but preservationists said it would have never happened had the cemetery been on the National Register of Historic Places, which grants protections to historic artifacts.

The urns were returned to the Army, which has said it plans to put them on display at a museum or perhaps the cemetery. It also is working to list Arlington on the National Register.

Over the years, the cracks, which run horizontally across the monument, have grown in length and width. They were repaired in 1975 and 1989, but in 2007, they were notably visible. One measured 28.4 feet long; another, 16.2 feet. That year the cemetery announced plans to replace the monument because of the cracks, saying that it was necessary to maintain Arlington’s pristine appearance.

Preservationists said that the monument could and should be repaired to maintain the authenticity of the Tomb, where a changing of the guard ceremony takes place every half-hour in the spring and summer. Ultimately, they prevailed, and in April 2010, the cracks were repaired.

Last fall, however, cemetery officials noticed that the grout used to fill the cracks was flaking and, in some cases, falling out.

Then last month, it notified several preservation groups that it planned to repair the monument June 6, and it asked for input on the project.

Comments were due May 31 — the day that Nieweg received the letter.

Because one day was not enough time to thoroughly comment on the project, Nieweg said Wednesday, he called the Army Corps of Engineers, which is to perform the work, to voice his objections.

“That’s not being transparent,” he said. “So we pushed back.”

Bruzese said that she was “sorry about the mailings.” She said the cemetery postponed the work and set up a conference call Tuesday with several parties, including the Trust, the National Park Service and the Virginia Department of Historic Resources.

“We were very pleased with the input we got from everybody,” she said.

That means experimenting with several types of grout and performing the work under the right conditions, which probably won’t be until September. Parts of the monument might have to be covered during the repairs, she said, but the Tomb won’t be closed to visitors.