Federal prosecutors say a former prison guard charged with killing two Tennessee post office workers planned to escape from custody by attacking a U.S. marshal.

In court documents filed Friday, the U.S. attorney's office in Memphis said Chastain Montgomery collected plastic trays from which he intended to make a "shank" in his cell at the federal detention center in Mason, as he was awaiting trial in the killings.

Prosecutors said Montgomery planned to attack a deputy U.S. marshal while being transported to Memphis for a court appearance. Montgomery allegedly attacked a corrections officer when found with the materials.

Montgomery has pleaded not guilty to fatally shooting Paula Robinson and Judy Spray while robbing the Henning post office with his 18-year-old son in October 2010.

The government is seeking the death penalty at trial.