The victims of the Jan. 8 shooting included then-Rep. Gabrielle Giffords (D-Ariz.), who was speaking to constituents at public event. Loughner had previously pleaded not guilty.
According to the Associated Press, a court-appointed psychiatrist will testify on Tuesday that Loughner is fit to stand trial. Loughner's guilty plea would result in a sentence of life in prison, a source told the AP.
Loughner has been charged with 49 counts, including murder and attempted assassination of a member of Congress. Among those killed was a federal judge, an aide to Giffords and 9-year-old girl.
Giffords suffered severe brain injuries that damaged her abilities to speak and move. After months of rehabilitation, she returned to Congress in August 2011 to vote on the debt ceiling bill. In January 2012, she announced her resignation.
The shooting has sparked a debate about gun laws in Arizona and across the nation. Critics say that Loughner, who had two previous offenses and a history of unusual behavior, should never have been able to purchase a gun. One survivor called the incident an "extremely tragic example of what is at stake each and every time a gun falls — or is placed — in the wrong hands."
Loughner is currently receiving treatment for mental illnesses at a U.S. Bureau of Prisons facility, where he will remain until a judge decides he is fit to stand trial. He is expected to enter his plea on Tuesday.