The gun was linked to the January 2013 slaying of Assistant District Attorney Mark Hasse outside a courthouse in Kaufman County, southeast of Dallas. A law enforcement official familiar with the investigation said a dive team found the weapon in Lake Tawakoni, 50 miles east of Dallas, earlier this year.
Two guns were found in the lake, concealed inside a black mask, the official said.
The official was not authorized to speak publicly on the ongoing case and spoke on condition of anonymity.
Two months after Hasse's killing, District Attorney Mike McLelland and his wife, Cynthia, were found shot dead in their home.
Former county Justice of the Peace Eric Williams and his wife, Kim, face capital murder charges in the deaths of Hasse, the district attorney and his wife. The two are scheduled for trial later this year.
Authorities believe Eric Williams bore a grudge against McLelland and Hasse for successfully prosecuting him for stealing three county-owned computer monitors. That conviction would cost Williams his job as a justice of the peace and his law license.
While investigators have found several weapons in a storage locker used by Eric Williams, those weapons had not been linked by testing to the crime before the dive team's search.
The Texas Department of Public Safety would not comment. A message left with the sheriff was not immediately returned.
WFAA-TV first reported the results of the dive team's search.
Follow Nomaan Merchant on Twitter at http://www.twitter.com/nomaanmerchant .