The writings found in Jared Lee Loughner's home read "I planned ahead," "My assassination," "Giffords" and "Die, B----."
Thirteen people, including Congresswoman Gabrielle Giffords, were wounded in the Jan. 8 shooting at an event where the congresswoman was meeting with her constituents.
Lawyers for Loughner are fighting the handwriting sample, saying it violates his Fifth Amendment right to remain silent.
Prosecutors say they want an FBI forensic examiner to instruct Loughner to copy certain letters, words, numbers or phrases onto paper for the examiner to make an analysis and reliable conclusion.
Autopsy results have been released for the six people killed.
They show that Christina-Taylor Green, the 9 year old born on Sept. 11, died of a shot to the torso that struck her heart and kidney.
Three others died of gunshot wounds to the head: Dorwin Stoddard, Phyllis Schneck and Gabe Zimmerman.
Loughner was charged Friday with the murders of U.S. District Judge John Roll and Giffords aide Zimmerman, and with causing the deaths of four others who were not federal employees, including Green.
The Associated Press contributed to this story.