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A bipartisan group of six senators urged Attorney General Eric Holder on Tuesday to pull the plug on his decision to send suspects in the Sept. 11 attacks to federal court, marking the second day in a row that senators from both sides of the aisle have accused the Obama administration of treating terror suspects like common criminals.

The senators on Tuesday urged the Department of Justice to try Khalid Sheikh Mohammed and other alleged Sept. 11 conspirators in military commissions and halt plans to try them in federal court in New York City. They warned that a federal trial could compromise classified evidence and serve as a recruitment tool for terrorists.

"The attacks of 9/11 were acts of war, and those who planned and carried out those attacks are war criminals," they wrote, expressing concern about the administration's use of the criminal justice system for "enemy combatants."

The letter was signed by Sens. Joe Lieberman, I-Conn.; Blanche Lincoln, D-Ark.; Jim Webb, D-Va.; Lindsey Graham, R-S.C.; Susan Collins, R-Maine; and John McCain, R-Ariz.

On top of that, two congressional Republicans - Graham and Virginia Rep. Frank Wolf -- say they plan to introduce legislation that will block the transfer of the Sept. 11 suspects to New York City by cutting off funding.

"The cost of moving a handful of people and trying them and keeping them in New York City until the appeals are out could cost up to a billion and perhaps even over a billion dollars. So it will be done through the appropriations process," Wolf said.

Graham said he's confident he can round up the votes. Democrats are particularly jittery following the election of Republican Scott Brown, who ran on a strong national security platform, to the U.S. Senate in Massachusetts last week.

"There are a lot of Democrats who oppose the idea of sending them to New York City, putting them in civilian courts. And if I have another vote, I think I'll get a majority of the United States Senate," Graham said.

Graham said he's concerned the administration continues to criminalize the war, and that the treatment of the alleged Christmas Day bomber -- who was read his Miranda rights hours after his capture -- underscores that point.

"This is not Nuremberg, trying people after the war is over. The war is still going on," he said. "I think the administration has made a mistake that could bring the administration down."

Graham is hardly the only lawmaker raising concerns about the way Christmas Day plot suspect Umar Farouk Abdulmutallab was handled following the attempted bombing of a Northwest Airlines flight on its way to Detroit.

Lieberman and Collins on Monday wrote a letter to Holder and counterterrorism adviser John Brennan complaining that the decision to read Abdulmutallab his rights shortly after his arrest was an opportunity lost and urging the administration to transfer him to military custody.

"The decision to treat Abdulmutallab as a criminal rather than (an unprivileged enemy belligerent) almost certainly prevented the military and the intelligence community from obtaining information that would have been critical to learning more about how our enemy operates and to preventing future attacks," the senators wrote.

They cited reports that Abdulmutallab was "speaking openly about the attack" and Al Qaeda in the Arabian Peninsula's involvement in it before he was read his Miranda rights.

Fox News has confirmed that local FBI agents interviewed the suspect for about 50 minutes after he was taken to the University of Michigan Medical Center. After he was advised of his rights later on, he obtained a lawyer.

Lieberman and Collins wrote that officials would be able to continue interrogating Abdulmutallab and try him before a military commission if they treat him as an enemy combatant.

White House Press Secretary Robert Gibbs had no comment when asked about the latest bipartisan letter Tuesday.

"I have not seen the letter," Gibbs said, tossing the question to the Justice Department.

When asked for comment, the Justice Department referred Fox News to Holder's opening statement when he testified before Congress last year. In the statement, Holder said he knew his decision to seek a civilian trial for the Sept. 11 suspects would be controversial, but he disputed claims that it would jeopardize classified information or provide a platform for the terror suspects. He noted that the department is pursuing other Sept. 11 trials via military commission, and said taken together the prosecutions represent the "ultimate steps toward justice" in the Sept. 11 attacks.

Fox News' Catherine Herridge contributed to this report.