FBI agents seized another cache of historical Civil War era documents yesterday, as well as $18,400 in cash, a loaded handgun and hundreds of capsules believed to be tranquilizers, from a safe deposit box maintained by Charles Merrill Mount, theD.C. author arrested last week after trying to sell rare letters apparently stolen from the National Archives.

The agents, according to records filed yesterday in U.S. District Court, found 194 historical documents, including manuscripts signed by Abraham Lincoln and Andrew Johnson and letters from Secretary of War Edwin Stanton to various Union generals.

It was the second such seizure of documents since Thursday, when Mount, 59, was arrested at a Boston bookstore after an attempted sale of rare letters written by Lincoln, Winston Churchill and others, which were allegedly stolen from the National Archives.

On Friday, the FBI here found 162 historical documents, identified as the property of the Library of Congress and valued at more than $100,000, in a safe deposit box at a downtown branch of the American Security Bank and Trust. The box was maintained by Mount but listed in the name of Edward Hearn.

Authorities have not placed a value on the documents seized yesterday, which were found in another safe deposit box at a Riggs National Bank branch next door to Mount's American Security bank box. The Riggs deposit box was under the name of Sidney Nussenbaum and listed Mount's Capitol Hill rooming house address.

At the National Archives and Library of Congress yesterday, staff members were busy with the laborious task of examining copies of the documents seized in Boston and cross-checking them with the historical collections at both institutions. Representatives of the two facilities were present Friday and yesterday when the FBI seized Mount's bank deposit boxes and will be examining those confiscated documents as well, according to officials.

Mount, meanwhile, was released on $50,000 bond yesterday in Boston, where he faces federal charges of interstate transportation of stolen property.

The federal complaint alleged that Mount took from Washington to Boston three Lincoln letters that were missing from the National Archives and nine letters written by 19th century artist James Whistler that were discovered missing from the Library of Congress, according to the FBI's Boston office.

Mount, an art historian who had fallen on hard times, was bailed out of jail by a longtime friend in Massachusetts. His court-appointed attorney, Charles McGinty, said yesterday that Mount would have no comment "at this time" on the allegations.

McGinty said last week that the government had "jumped the gun" in arresting Mount and that authorities would have trouble proving that the documents in his possession were the same ones missing from the Library of Congress and the National Archives.

An FBI inventory of documents and other items seized yesterday listed, in addition to actual historical papers, an empty folder marked, "Stanton on Lincoln's death." The gun was a .32-caliber handgun "that was loaded," according to one law enforcement source. In addition, 926 capsules believed to be tranquilizers and a can of Mace were confiscated, authorities said.

Mount, once a promising author and portraitist who grew up in Brooklyn but affected a British accent, frequently researched records at the Library of Congress and the

National Archives, according to officials there.

"He made many, many visits and looked at a wide range of records, including those from the Civil War and World War I," said Jill Brett, a spokeswoman for the National Archives. "We have slips with his name going back to 1984."

Brett said the number of documents on file, about 3 billion pieces of paper, prevents each item from being catalogued individually. Instead, the items are numbered and filed by subject and record group in archival boxes that contain as many as 1,000 pieces of paper. These boxes are now being checked to see if numbers are out of sequence.

"We're going into boxes and checking to see what's missing," Brett said, adding that "a cursory appraisal" so far indicates that 165 of the documents confiscated in Boston were previously in the National Archives' possession.

Although they do not make body searches, officials at the Library of Congress and the National Archives have described elaborate security precautions they say are used to protect their collections. These include keeping guards in the research rooms, prohibiting purses and briefcases, distributing special paper for note-taking and checking all hand-held papers and returned boxes.

As a result of the Mount case, however, Brett said the building is "reevaluating both security and research procedures."

A spokeswoman at the Library of Congress said the library is working to document "evidence of L.C. ownership of materials found in his possession," but she declined to discuss specific procedures "since the case is an active one."