WASHINGTON (CNN) -- Democrats forced the Senate into a closed session Tuesday to pressure the Republican majority into completing an investigation of the intelligence underpinning the 2003 invasion of Iraq.Democrats demanded that Intelligence Committee Chairman Pat Roberts move forward on a promised investigation into how Bush administration officials handled prewar intelligence about Iraq's suspected weapons programs.The probe would be a follow-up to the July 2004 Intelligence Committee report that blamed a "series of failures" by the CIA and other intelligence agencies for the mistaken belief among U.S. policymakers that Iraq had restarted its chemical, biological and nuclear weapons programs. (Full story)The Senate reopened about two hours later, after members agreed to appoint a bipartisan group of senators to assess the progress of the "Phase 2" probe, the office of Majority Leader Bill Frist said. (See video on Democratic move -- 3:05)The three Republicans and three Democrats are to report back to Senate leaders by November 14.Democrats accused Roberts of stalling the probe into how administration officials handled the intelligence used to sell Congress and the public on invading Iraq.Roberts, a Kansas Republican, said the closed session was "not needed, not necessary and, in my personal opinion, was a stunt."The closed session was punctuated by acrimonious broadsides in the Capitol hallways.Frist said Democrats had "hijacked" the Senate, and Democrats threatened to close the chamber each day until Republicans agreed to move forward with the investigation."This is an affront to me personally," said Frist, a Tennessee Republican. "This is an affront to our leadership. It is an affront to the United States of America, and it is wrong."Frist said Senate Rule 21 -- which requires everyone but senators and a few aides to clear the chamber until a majority votes to reopen -- had been invoked only rarely and with "mutual conversation" between the leaders of both parties. Democratic leader Harry Reid said the surprise move was necessary to overcome Republican efforts to "obstruct" a full investigation of how the Bush administration led the United States into war."There's nothing more important to a Congress or a president than war," the Nevada Democrat said. "I think the American people are entitled to know how we got there. That's what this is all about."There was no immediate reaction from the White House.Reid said the GOP leadership in Congress has "repeatedly chosen to protect the Republican administration rather than get to the bottom of what happened and why." He said he had "zero regret" about the move: "The American people had a victory today."Rule 21 has been invoked 53 times since 1929, according to the Congressional Research Service. It was invoked six times during the impeachment trial of former President Bill Clinton for senators to organize the proceedings and deliberate on his eventual acquittal.Roberts: Probe in progressSen. Jay Rockefeller, the intelligence committee's ranking Democrat and vice chairman, said the Democratic maneuver was necessary for Americans to learn who was accountable for the way prewar intelligence was used."Everything is about accountability to the American people, accountability of the executive branch ... [and] accountability of the oversight of the Congress," Rockefeller said.He said the committee's Republican majority has refused to request documents from the White House about how the Bush administration crafted arguments for the invasion."What disturbs me the most is the majority has been willing, in this senator's judgment, to take orders from this administration when it comes to limiting the scope of appropriate, authorized and necessary oversight investigations," Rockefeller said.Roberts said his committee has been working on the Phase 2 investigation since May and "we have what we think is a pretty good report." He said the committee will take up the matter next week."However long it takes, working in good faith, we will look into Phase 2 and see what we can do and finish that product," Roberts said.Sen. Carl Levin, a Michigan Democrat on the panel, expressed his doubts. "Assurances have been made for months that progress is being made," Levin said. "We have not seen any evidence of it."Democrats last year had pushed for the second part of the panel's inquiry to be completed before the November 2004 elections.Democratic Whip Richard Durbin said last week's indictment of Vice President Dick Cheney's top aide, I. Lewis "Scooter" Libby, on perjury and obstruction of justice charges showed how the Bush administration reacts to criticism. Libby is accused of lying to investigators and a grand jury probing the disclosure of the identity of a CIA officer whose husband had challenged a key assertion in the administration's case for war."It's a question about whether or not anyone in this administration in any way misused or distorted intelligence," Durbin said. He said senators "owe the American people some straight answers."Durbin, an Illinois Democrat, denied his party was trying to stall Senate action on Supreme Court nominee Samuel Alito.He said work on Alito's nomination was still going on, and he was scheduled to meet with the nominee on Wednesday.Sen. Christopher Bond of Missouri, a Republican member of the Intelligence Committee, said Democratic complaints against Roberts were "terribly unfair and unfounded." Bond said the panel's 2004 report found no indication that the mistaken assumptions about Iraq's weapons programs were the result of political pressure."Even after they signed on to that, they contend that somehow this intelligence was misused," he said.Responding to that argument, Durbin told CNN, "This is a different question: Once they received the intelligence, did members of the administration accurately and honestly portray it to the American people?"CNN's Ted Barrett contributed to this report.