But a federal judge ruled Thursday that prosecutors must present evidence that the additional time is necessary to restore Jared Lee Loughner to competency.
Loughner's evaluation period at the prison in Springfield, Mo., is scheduled to end Sept. 26.
Prosecutors want to extend his treatment through Jan. 26.
U.S. District Judge Larry Burns set a Sept. 21 hearing by video teleconference from San Diego on the issue.
Loughner has been in Missouri since May 27 after being declared mentally unfit to stand trial.
He has pleaded not guilty to 49 charges in the Jan. 8 shooting that killed six and wounded 13, including Rep. Gabrielle Giffords.
(Copyright 2011 by The Associated Press. All Rights Reserved.)