Updated

A man charged with killing four people in a shooting rampage that began in a courthouse used his own blood to sign letters that detailed his plot to escape from a county jail with another inmate, prosecutors said Tuesday.

The disclosure was made in a court filing in which prosecutors asked a judge to allow the other inmate's testimony. Prosecutors said there is no evidence that Steven Marshall is not telling the truth about his conversations with accused gunman Brian Nichols.

"Nor is it conceivable that Marshall devised a plot to entrap defendant into writing letters detailing his proposed escape, gathering items such as a contraband cell phone to assist in the scheme, and signing the letters in his own blood," the prosecutors wrote.

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The filing did not elaborate on the contents of the letters.

Prosecutors say Nichols plotted an escape with other inmates, including Marshall, at the jail where he has been held since the March 11, 2005, shootings at the Fulton County Courthouse. There's been no indication thus far that an escape attempt actually occurred.

Defense lawyers said Marshall, who has a lengthy criminal record that includes charges of armed robbery and murder, is an unreliable witness, and suggested that prosecutors placed Marshall in the cell next to Nichols to get Nichols to further incriminate himself.

But prosecutors disagreed, saying Nichols involved Marshall in the case and that Marshall was the one who alerted authorities about Nichols' statements through his attorney and family members.

Nichols is accused of grabbing a deputy's gun at the courthouse, where he was being tried on rape charges, and killing a judge, court reporter and a sheriff's deputy. He's also accused of killing a federal agent he encountered at a home a few miles from the courthouse that night.

Police said Nichols also took a woman hostage in her suburban Atlanta apartment but surrendered the next day.

Prosecutors are seeking the death penalty. His trial on the murder charges is scheduled to begin Jan. 11.