The Associated Press

SEATTLE (AP) - A former Army Ranger who led a terrifying takeover-style bank holdup has pleaded not guilty to new charges - that he tried to kill a fellow robber behind bars.

Luke E. Sommer, 22, entered the plea Thursday to charges of assault with intent to commit murder and assault with a deadly weapon.

Prosecutors say he used an improvised plastic knife to attack 20-year-old Nathan R. Dunmall of Chilliwack, British Columbia, in January at the Federal Detention Center in SeaTac.

Sommer, a U.S.-Canadian dual citizen, Dunmall and three others are serving time for the $54,000 holdup of a Bank of America branch in Tacoma on Aug. 7, 2006.

According to investigators, Sommer attacked Dunmall for suspected cooperation with the government.

Trial on the new charges is set for Sept. 28.