BALTIMORE — A presidential historian’s assistant pleaded guilty Thursday to conspiring to steal valuable documents from archives throughout the Northeast.

Jason Savedoff, 24, entered the plea in U.S. District Court in Baltimore.

Savedoff and historian Barry Landau were charged in July with stealing historical documents from the Maryland Historical Society and conspiring to steal documents from other archives. Both initially pleaded not guilty.

Prosecutors alleged that Landau and Savedoff had about 80 documents when they were arrested at the historical society in Baltimore in July. About 60 belonged to the society, including papers signed by President Abraham Lincoln worth $300,000 and presidential inaugural ball invitations and programs worth $500,000....