SEATTLE — A former Army Ranger who led a terrifying takeover-style bank holdup pleaded not guilty Thursday to new federal charges that he tried to kill a fellow robber behind bars because he thought the one-time colleague had ratted him out.

Luke E. Sommer, 22, nodded, laced his fingers and said nothing as his lawyer entered the plea in U.S. District Court to charges of assault with intent to commit murder and assault with a deadly weapon. Trial was set for Sept. 28.

Sommer, who served in Iraq and Afghanistan and is described in court documents as having bipolar disorder and possibly post-traumatic stress syndrome, is accused of attacking Nathan R. Dunmall, 20, of Chilliwack, B.C., in January.

Sommer sharpened a chunk of plastic from a stairstep exercise machine to make an improvised knife before the attack at the Federal Detention Center in SeaTac, authorities said.

Investigators wrote that Sommer led a small band of Rangers that planned to establish a crime organization that could challenge the Hells Angels motorcycle gang in British Columbia when they made off with $54,000 from a Bank of America branch in Tacoma on Aug. 7, 2006.

The heist was done “with military style and planning,” an FBI agent wrote in court documents. Government lawyers described it as “one of the most dangerous bank robberies ever committed in Washington.”

Sommer entered the bank with three others, leaped over the counter, trained a semiautomatic handgun with a laser sight on the frightened tellers and demanded money. All were wearing Army-issue soft body armor. They were gone within two minutes and 21 seconds.

“Sommer and his gang were prepared for combat,” Assistant U.S. Attorneys Michael Dion and Jill Otake wrote. “If the police had shown up during the robbery, there would have been a bloodbath on the streets of Tacoma.”

Sommer, a U.S.-Canadian dual citizen who was once stationed at Fort Lewis, was later arrested in Canada. He told authorities he had planned to use the heist to draw attention to a rape he said he witnessed in Iraq in 2004 and the killing of civilians by Navy SEALs in Afghanistan in 2005, but Army investigators said his accounts could not be substantiated.

Sommer, Dunmall and three others were sentenced to prison.

Defense lawyer Steven John Krupa and Assistant U.S. Attorney Gregory A. Gruber wouldn’t say why Sommer and Dunmall were in detention — akin to a jail — rather than in prison, so long after being sentenced. They also wouldn’t comment on whether Sommer had been given a mental evaluation or was taking medication.

According to investigators, Sommer believed Dunmall had cooperated with the government. He spent 60 days planning the attack and managed to enter Dunmall’s cell during a visit to the medical area, authorities said.

Dunmall was able to defend himself and guards separated the pair before either was badly injured.

A counselor at the lockup told Treat he heard Sommer yell at Dunmall that he wouldn’t be safe even if were sent to the ADX, a federal super-maximum security prison in Florence, Colo.

“You can’t hide anywhere,” Sommer said, according to the counselor.

Sommer is now confined in SeaTac’s special housing unit, “essentially a lockdown area,” Gruber said.