Art historian Charles Merrill Mount was arrested by FBI agents at his Capitol Hill rooming house yesterday morning and charged with receiving government property, specifically a letter by novelist Henry James that is said to belong to the Library of Congress.

The three-page letter, signed by James and written in 1904, was found last week in a safety deposit box that Mount maintained at a downtown bank, according to court records. At a news conference yesterday, FBI officials said the letter had been authenticated by the Library of Congress and is valued at more than $5,000.

The FBI said the letter was among more than 200 historical and other documents seized by law enforcement officials during a search of Mount's residence and two bank deposit boxes he maintained under other names. Though most of these documents have not yet been authenticated, an FBI official said many are believed to be the property of the Library of Congress and the National Archives.

Mount, 59, had been arrested last Thursday by FBI agents in Boston after he attempted to sell rare letters written by Abraham Lincoln, Winston Churchill and others and allegedly stolen from the National Archives. Mount faces federal charges of interstate transportation of stolen property stemming from that arrest.

The author and portraitist, who apparently had fallen on hard times in recent years, was released on $50,000 bond in Boston Monday and returned that night to Washington, according to his court-appointed attorney in Boston. Mount was arrested by FBI agents here about 11:30 a.m. yesterday.

"Basically, this is an intrusion into the history of this country," said W. Douglas Gow, special agent in charge of the FBI's Washington field office. Mount's possession of the James letter, Gow said at a news conference, was keeping "a piece of history" that would be useful to other scholars.

A spokeswoman for the Library of Congress said the letter, part of the library's Henry James collection, was correspondence from James to 19th century artist James Abbott McNeill Whistler, best known for the portrait of his mother.

Mount, who grew up in Brooklyn as Sherman Suchow, but later changed his name and affected an English accent, was arraigned in federal court yesterday and ordered held in lieu of a $25,000 cash bond. He was dressed nattily -- gray slacks, black and white checked sports coat, white handkerchief -- but without his customary cane.

In court, prosecutors said Mount had served four months in prison after being convicted of making threatening telephone calls to his mother. Mount also was placed on a year's probation after he was found guilty of filing false statements in an application for a passport. In addition, according to prosecutors, Mount was once convicted in England of car theft.

In arguing for a high bond, prosecutors expressed concern that Mount might flee the country, and noted that he had used at least three aliases: Jack Brody, Lynn Proby and Edward Hearn. They also said that keys to two other safety deposit boxes had been found during a search of Mount's home, leading prosecutors to believe that more historical documents may be in Mount's possession.

So far, according to authorities, the locations of these additional boxes have not been determined.

The James letter, according to an FBI arrest warrant affidavit, was in a safety deposit box at a downtown D.C. branch of the American Security Bank and Trust. Authorities said the box was maintained by Mount under the name of Edward Hearn, and was searched last week by FBI agents.

Also last week, according to one FBI agent, authorities found a "medium sized" painting bearing the signature of French artist Claude Monet, standing against a wall in Mount's room. The agent said the FBI was trying to determine if the painting is authentic.

On Monday, FBI agents opened another safety deposit box that Mount maintained at a branch of the Riggs National Bank, next door to the American Security Bank where he kept the first box.

The second box was in the name of Sidney Nussenbaum, according to the FBI, and contained Civil War era documents, more than $18,000 in cash, a loaded handgun and a number of capsules believed to be tranquilizers.

In court yesterday, Mount said the tranquilizers belonged to his late father, and that he had put them in the bank box so "they would not be in harm's way."

Mount, who reportedly had $19 in his pocket when arrested in Boston, said, "I have no other funds of any kind."

In the arrest affidavit, the FBI said some of the manuscripts Mount was trying to sell in Boston have been identified through reference catalogues as three original letters signed by Lincoln and belonging to the National Archives. The FBI, citing statements from a Boston woman, said Mount had sold other original manuscripts "that have been subsequently identified as having been stolen from the Library of Congress."

The FBI affidavit said Mount had visited the archives more than 40 times and was known as a Civil War buff. Officials at the Library of Congress said the art historian donated to the library research materials for several biographies he had written and had had his own "working space" in a library office for the last year and a half.

"He was somebody we trusted," said one official in the library's manuscript division.