Senate Armed Service Committee Chairman Sen. Carl Levin D-Mich. shakes hands with outgoing Defense Secretary Leon Panetta on Capitol Hill in Washington, Thursday, Feb. 7, 2013, prior to Panetta testifying before the committee's hearing on the Pentagon's role in responding to the attack last year on the U.S. consulate in Benghazi, Libya, where the ambassador and three other Americans were killed. Entering the hearing room are Sen. Lindsey Graham, R-S.C., left, and Sen. Joe Manchin, D-W. Va., second from left. WASHINGTON (AP) - The U.S. military is determined to position small, quick reaction forces closer to global crises after the rapid assault on the U.S. diplomatic mission in Libya last September kept U.S. armed forces from responding in time to save four Americans. Defense Secretary Leon Panetta and Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told Congress on Thursday that they moved quickly to deploy commando teams from Spain and Central Europe last Sept. 11, the chaotic day of the assault on the U.S. installation in Benghazi, but the first military unit didn't arrive until 15 hours after the first of two attacks. "Time, distance, the lack of an adequate warning, events that moved very quickly on the ground prevented a more immediate response," Panetta said in likely his last Capitol Hill appearance before stepping down as Pentagon chief. Republicans have accused the Obama administration of an election-year cover-up of a terrorist attack in the nearly five months since the assault, and they kept up the politically charged onslaught on Thursday. The military also found itself under attack, with at least one senator accusing the Joint Chiefs chairman of peddling falsehoods. Faced with repeated questions about where units were during the attack and what they were doing, Dempsey said the military is taking steps to deal with the next crisis. "We've asked each of the services to examine their capability to build additional reaction-like forces, small, rapidly deployable forces," Dempsey said. A small MAGTF for the Marine Corps, for example, a Marine air-ground task force. And the Army is looking at some options as well to increase the number of these resources across the globe, where the limiting factor, though will always be basing. Sen. Angus King, I-Maine, put it in layman's terms: "So you are moving the fire stations nearer the ...?" We're trying to build more firemen. The question is whether I can build the stations to house them," Dempsey answered. In more than four hours of testimony, Panetta and Dempsey described a military faced with not a single attack over several hours, but two separate assaults six hours apart; little real-time intelligence data and units too far away to mobilize quickly. Ambassador Chris Stevens and three other Americans were killed in the attacks. Between midnight and 2 a.m. on the night of the attack, Panetta issued orders, telling two Marine anti-terrorism teams based in Rota, Spain, to prepare to deploy to Libya, and he ordered a team of special operations forces in Central Europe and another team of special operations forces in the U.S. to prepare to deploy to a staging base in Europe. The first of those U.S. military units did not actually arrive in the region until well after the attack was over and Americans had been flown out of the country. Just before 8 p.m., the special operations team landed at Sigonella Naval Air Station in Sicily. An hour later, the Marine team landed in Tripoli. Defense officials have repeatedly said that even if the military had been able to get units there a bit faster, there was no way they could have gotten there in time to make any difference in the deaths of the four Americans. "The United States military is not and should not be a global 911 service capable of arriving on the scene within minutes to every possible contingency around the world," Panetta told the Senate Armed Services Committee. That failed to placate Republicans on the panel. In one fierce exchange, Sen. John McCain, R-Ariz., called Dempsey's statement "one of the more bizarre" and argued that if the administration had taken security threats seriously, aircraft and other military could have been located at Souda Bay, Crete. "For you to testify before this committee that ... consistent with available threat estimates is simply false; that our military was appropriately responsive," McCain said. What would have been an inappropriate response since ... no forces arrived there until well after these murders took place? The general said the military was concerned with multiple threats worldwide and, based on time and positioning of forces, "we wouldn't have gotten there in time." Several committee Republicans pressed Panetta and Dempsey about their discussions with President Barack Obama on that fateful day and his level of involvement, suggesting that after the initial conversation the commander in chief was disengaged as Americans died. Panetta said he and Dempsey were meeting with Obama when they first learned of the Libya assault. He said the president told them to deploy forces as quickly as possible. Sen. Lindsey Graham, R-S.C., questioned whether Panetta spoke again to Obama after that first meeting. The Pentagon chief said no but that the White House was in touch with military officials and aware of what was happening. During the eight-hour period, did he show any curiosity? Graham asked. Panetta said there was no question the president was concerned about American lives. Exasperated with Graham's interruptions, Panetta said forcefully, "The president is well-informed about what is going on; make no mistake about it." At one point in the hearing, Graham asked Panetta if he knew what time Obama went to sleep that night. The Pentagon chief said he did not. Panetta also pushed back against Republican criticism that the Obama administration ignored warning signs about the attack. The Pentagon chief insisted there were no signs of or specific intelligence about an imminent attack. In the six months prior to the assault, the government was apprised of 281 threats to diplomatic missions, consulates and other facilities worldwide, he said. Sen. Kelly Ayotte, R-N.H., pressed Dempsey on why F-16 jets in Aviano, Italy, weren't sent to Libya. Dempsey said it would have taken up to 20 hours to get the planes ready and on their way, and he added that they would have been the "wrong tool for the job." Panetta later explained to the committee, "You can't willy-nilly send F-16s there and blow the hell out of place. ... You have to have good intelligence. Sen. Saxby Chambliss, R-Ga., asked whether Panetta and Dempsey would describe the Benghazi incident as an "intelligence failure." Panetta stopped short of using that term, saying simply that "some of the initial assessments were not on the money." Dempsey called it an "intelligence gap." Sen. James Inhofe, the committee's top Republican, criticized the administration for trying to "cover up" what he said was clearly a terrorist attack. The U.S. ambassador to the United Nations, Susan Rice, initially attributed the violence to a protest against an American-made, anti-Islam video. Rice's comments touched off a deeply partisan feud, with Republicans claiming the Obama White House wanted to obscure the reasons for the incident to help the president's re-election bid. The criticism of Rice was largely responsible for scuttling her chances to become secretary of state. Panetta is retiring after a Washington career that has stretched across four decades, with years as a California congressman, budget chief, White House chief of staff to President Bill Clinton and CIA director who oversaw the hunt for and killing of terrorist leader Osama bin Laden. The Defense Department will bid farewell to Panetta, who has served as defense secretary since June 2011, in a ceremony on Friday. Obama has nominated former Republican Sen. Chuck Hagel to succeed Panetta. Associated Press writer Lolita C. Baldor contributed to this report.