FBI agents discovered yesterday a new cache of historical documents, including letters signed by Ulysses S. Grant, in the safe deposit box of a Washington author arrested after trying to sell rare letters apparently stolen from the National Archives.

According to records filed yesterday in U.S. District Court, the FBI found 162 documents, many identified as "original Civil War era manuscripts" and others identified as the property of the Library of Congress, in a bank safety deposit box maintained for Charles Merrill Mount. An affidavit filed in court said the seized documents were valued at more than $100,000.

Meanwhile, Mount, a 59-year-old art historian who had fallen on hard times in recent years, remained in custody in Boston on federal charges of interstate transportation of stolen property. Mount's bond was set at $50,000 by a federal magistrate there, but the lateness of the hearing delayed his release, according to his attorney.

Mount was arrested Thursday at a Boston bookstore after, officials said, he tried to sell rare letters written by Abraham Lincoln, Winston Churchill and others, which were allegedly stolen from the National Archives.

A federal complaint against him alleged that Mount took from Washington to Boston three Lincoln letters that were missing from the National Archives and nine letters written by the 19th century artist James Abbott McNeill Whistler that were discovered missing from the Library of Congress, according to James F. Ahearn, special agent in charge of the Boston FBI office.

However, Mount's court-appointed lawyer, Charles McGinty, said yesterday, "There's a long road the government has to hoe to establish that these documents ever were in the Library of Congress or in the National Archives."

In a telephone interview, McGinty said the government "jumped the gun here. At the hearing today, they didn't have the kind of verification that the documents he had in his possession were the same documents {missing from the library and the archives}."

Neither Library of Congress nor National Archives officials could verify yesterday that the documents in question were those missing from the institutions. Jill Brett of the National Archives said archivists were awaiting copies of the documents from Boston to begin the examination process.

Brett said the archives is unlikely to go beyond the security measures already in place, but a Library of Congress official said that the library is reviewing its security measures for possible improvements.

U.S. Attorney Joseph E. diGenova described the case yesterday as an investigation "into the theft of history." DiGenova said the ownership of the documents seized in Washington will be investigated. "We simply don't know {where they came from}. We have a lot of questions right now."

According to court documents, FBI agents searched two safe deposit boxes at the American Security Bank, 1501 Pennsylvania Ave. NW, registered in the name of "Edward Hearn."

A Library of Congress employe, who said he had worked as a research assistant to Mount, told agents that Mount once showed him a safe deposit box at the bank, and the employe "noticed that Mount used the name 'Edward Hearn' to gain access to the vault," according to an affidavit filed in court. The library employe questioned Mount "about the phony name and was not provided a satisfactory answer," the affidavit said.

FBI agents also searched Mount's room in a boardinghouse at 114 Fifth St. NE, according to another affidavit, and found "file folders" containing "documents and manuscripts of historical value," "assorted photographs . . . marked Library of Congress," and "one canvas painting marked 'Claude Monet' 1907."

The arrest of Mount in Boston and the Washington searches were set in motion this week by Boston bookstore owners who became suspicious of the origin of the rare letters Mount had sold them last month. They contacted the FBI, which called in a rare documents specialist who identified some of the Whistler letters as ones he had sold earlier to the Library of Congress.

Officials of the library and the National Archives yesterday cautioned that they had not completed inventories of precisely what documents were missing from their original manuscript collections.

However, according to an affidavit in support of a search warrant filed yesterday in U.S. District Court here, at least 35 documents written by the artist Whistler, best known for a portrait of his mother, are missing from the Library of Congress collection.

On July 23, Mount sold 25 documents, including at least nine Whistler letters, to Goodspeed Bookstore on Beacon Street in Boston for $20,000, according to federal law enforcement officials.

The affidavit also said that three letters of Abraham Lincoln seized from Mount on Thursday in Boston "were identified from a historical manuscripts catalogue as being property of" the archives.

Since 1984, Mount has studied 630 bound volumes and 132 archival boxes full of mostly Civil War and World War I era military and diplomatic documents, according to Brett, the spokeswoman for the National Archives. In registering for access to the archives' 1930s-style second-floor reading room, Mount had listed himself as "a Civil War buff," Brett said.

David Wigdor, acting chief of the Library of Congress' manuscript division, said that for the past two years, Mount had spent several days a week in the closely guarded first-floor reading room for original manuscripts in the James Madison Building.

Wigdor said that when Mount registered as a scholarly reader with the library in June 1985, he said he was studying the Victorian painter John Singer Sargent, and wanted to read letters of artists such as Whistler who were Sargent's contemporaries.

Both collections struggle for "a balance between access and security" and are "among the most secure" collections of original manuscripts in the world, said Donn Neal, executive director of the Society of American Archivists.

Both collections have elaborate systems of choosing and registering scholars who are allowed access to reading rooms that contain original manuscripts. Readers are supervised by staff members and guards. Coats, purses and briefcases are checked into lockers or with guards before readers enter the rooms.

Readers are allowed to bring one pencil, issued by the library or the archives, into the room. Readers with special permission may also bring in a typewriter or camera, which Mount never did, according to Brett. Readers are "visually searched" by guards as they leave and must sign out.

"Do you really want to get into strip-searching serious researchers? It's a very difficult dilemma for archives or repositories of manuscripts," Brett said. "You don't want to impugn the motives of everyone who comes here. It's a risk, but I suppose we'll continue taking it, very mindful of the negative aspects of it." Staff writer Marcia Slacum Greene contributed to this report.