Washington (CNN) -- Attorney General Eric Holder defended his decision Wednesday to try five suspected 9/11 terrorists in civilian court. Faced with bitter criticism from Republican senators at a contentious hearing, he insisted his decision reflects his conviction that a civilian trial represents the best chance for a successful outcome.
"I have every confidence that the nation and the world will see him for the coward that he is," Holder said, referring to confessed 9/11 mastermind Khalid Sheikh Mohammed. "Failure (at trial) is not an option. These are cases that have to be won," the attorney general declared.
Skeptical Republicans on the Senate Judiciary Committee pounced on the claim, saying a judge and jury will make that decision, not Holder and his prosecutors.
During tough questioning from Sen. Jon Kyl, R-Arizona, spontaneous applause erupted from angry family members of 9/11 victims who wanted the suspects tried in a military court.
"How could you be more likely to get a conviction in federal court when Khalid Sheikh Mohammed has already asked to plead guilty before a military commission, and be executed?" Kyl demanded.
Holder rejected GOP charges the Obama administration refuses to acknowledge the U.S. is at war.
"We are at war, and we will use every instrument of national power -- civilian, military, law enforcement, intelligence, diplomatic and others -- to win," he told members of the committee. "We need not cower in the face of this enemy."
Holder said he knew his decision would be controversial when he announced last week that the suspected terrorists will be tried in civilian court in New York City.
All five suspects have been held in the U.S. detention facility at Guantanamo Bay, Cuba.
Critics charge Holder is simply trying to treat hardened terrorists as common criminals.
"There is nothing common about the treatment the alleged 9/11 conspirators will receive," Holder promised. "In fact, I expect to direct prosecutors to seek the ultimate and most uncommon penalty for these heinous crimes."
The attorney general has promised to seek the death penalty for all five suspects.
Vermont Sen. Patrick Leahy, the chairman of the Judiciary Committee, said at the start of the hearing that he agrees with Holder's decision.
The alleged terrorists "should answer for their brutality," he said. "I have full confidence in the capacity of New York" to try the case, he said.
Federal courts have successfully tried more than 100 terrorist cases since the September 11 attacks, Leahy said.
Alabama Sen. Jeff Sessions, R-Alabama, questioned Holder's judgment. Sessions argued the decision to try the five suspects in a civilian court was one of many Obama administration actions signaling to allies that "fighting global terrorism is not the priority it once was."
Holder said he knows "we are at war with a vicious enemy who targets our soldiers on the battlefield in Afghanistan and our civilians on the streets here at home. ... Those who suggest otherwise are simply wrong."
Dozens of family members of 9/11 victims recently signed a letter to Holder and President Obama opposing a civilian trial for the alleged plotters.
One of them directly confronted Holder immediately after the hearing was adjourned.
"We are heartsick and weary of the delays and the machinations, and I am afraid that the theatrics are going to take over at this point, and I very much regret that," Alice Hoagland told the attorney general. Hoagland's 31-year-old son Mark Bingham was killed when hijacked United Airlines Flight 93 crashed near Shanksville, Pennsylvania.
Holder told her he believed a New York trial will bring justice the families deserve, and that the suspects would have no greater platform than they would enjoy at a military trial.
Holder sought to allay fears by critics that if Mohammed and the other suspects were for any reason not convicted at trial they could be released in the United States.
"We would continue to hold them under the laws of war," Holder said. "We believe we have the authority to do that," he said.