PHILADELPHIA — A 40-year-old intern with the National Archives pleaded guilty yesterday to stealing 164 Civil War documents, including an official announcement of President Lincoln’s death, and putting most of them up for sale on eBay.

Prosecutors said Denning McTague, who has master’s degrees in history and library science, put about 150 of the documents online and had sold about half of them.

All but three of the items, worth an estimated $30,000 in all, have since been recovered.

McTague told investigators that he used a yellow legal pad to sneak the documents out while working at the National Archives and Records Administration last summer. As an unpaid intern, he had been responsible for arranging and organizing documents in preparation for the upcoming 150th anniversary of the Civil War.

A Gettysburg company that publishes books on the Civil War spotted some of the items on eBay and alerted authorities last fall, officials said.

The stolen Civil War-era documents included telegrams concerning the troops’ weaponry, the War Department’s announcement of Lincoln’s death sent to soldiers, and a letter from famed cavalryman James Ewell Brown Stuart, prosecutors said.

McTague pleaded guilty to one federal count of stealing government property. U.S. District Judge Stewart Dalzell set his sentencing for July 12. The maximum sentence would be 10 years in prison and a $250,000 fine, but federal sentencing guidelines call for much less.

Paul Brachfeld, inspector general for the National Archives, said the documents are invaluable and getting them back wasn’t easy, especially since some had been sold overseas.

“They’re all the history of our democracy,” he said.