CIA Director Porter Goss resigned suddenly, nudged out after a turmoil-filled 19 months at the spy agency as it struggled to forge a new identity in an era of intelligence blunders and government overhauls. Air Force Gen. Michael Hayden, top deputy to National Intelligence Director John Negroponte, is the leading candidate to replace Goss, a senior administration official said. An announcement could come as early as Monday. Hayden served as National Security Agency director until becoming the United States' No. 2 intelligence official one year ago. Since December, he has aggressively defended the Bush administration's warrant-less surveillance program. Hayden was one of its chief architects. Goss offered little explanation for his resignation in a brief appearance Friday with President George W. Bush, a televised address to agency personnel and a written statement. "CIA remains the gold standard," he said. "When I came to CIA in September of 2004, I wanted to accomplish some very specific things, and we have made great strides on all fronts." But the agency, as well as the Bush administration, has been far from peaceful. Goss' departure was the White House's third major personnel move in just over a month, aimed at reinvigorating Bush's second term. Goss said he was willing to stay a while for a smooth transition, but there also was talk that an acting chief could be named. Knowledgeable Republicans said Friday night that Hayden was thought to top Bush's short list of candidates to replace Goss. Among others mentioned were Bush's homeland security adviser, Frances Fragos Townsend; David Shedd, Negroponte's chief of staff; and Mary Margaret Graham, Negroponte's deputy for intelligence collection. Making Friday afternoon's announcement from the Oval Office, Bush said Goss' tenure had been one of transition. The director, a former CIA operative and then Florida congressman, had been given the job only a little over a year and a half ago. The president said, with Goss at his side, "He's instilled a sense of professionalism. He honors the proud history of the CIA, an organization that is known for its secrecy and accountability." It was not entirely clear why Goss resigned so unexpectedly. An intelligence official, speaking only on condition of anonymity because of the sensitivity of his position, said Goss had stood up for the agency when there were differences with National Intelligence Director John Negroponte's office, which was created about a year ago. Goss was taking a stand against "micromanagement," the official said, and wanted the agency to "remain what its name says, the 'Central' Intelligence Agency." With the backing of the White House, Negroponte recently raised with Goss the prospect that he should leave, and the two talked about that possibility, a senior administration official said. That official also spoke on condition of anonymity, in order to give a fuller account of events. Negroponte, Goss' classmate at Yale University, said in a statement that Goss worked tirelessly during a CIA transition period. "As my friend for almost 50 years, I will miss Porter's day-to-day counsel," he said. Agency officials dismissed suggestions that the resignation was tied to controversy surrounding the CIA's executive director, Kyle "Dusty" Foggo. The FBI is investigating whether Foggo's longtime friend, defense contractor Brent Wilkes, provided prostitutes, limousines and hotel suites to a California congressman who pleaded guilty to taking bribes from Wilkes and others in exchange for government contracts. CIA spokesman Paul Gimigliano said Goss' resignation also was not related to the recent firing of a CIA officer the director said had unauthorized contacts with the press - a firing that found support within the agency and the White House. Congressman Darrell Issa, a Republican member of the House of Representatives' Intelligence Committee, said many in Washington want to know the full story. "I suspect that his decision could be based on any number of things that weren't stated, including a strong desire just to get on with his personal life after many years of public service," Issa said. Bush nominated Goss in 2004, in the midst of a re-election campaign that was riddled with accusations about the botched prewar intelligence on Iraq. Bush said he would rely on the advice of Goss on the sensitive issue of intelligence reform. Goss, the former chairman of the House Intelligence Committee, came under fire almost immediately, in part because he brought with him several top aides from Congress who were considered highly political for the CIA. They developed particularly poor relations with segments of the agency's clandestine service. By December, Congress passed the most sweeping intelligence overhaul in 50 years. One result: The CIA that took pride in being the premier element of the spy community found itself relegated to a crowded second tier of 15 other agencies. Congresswoman Jane Harman, the Intelligence Committee's top Democrat, said CIA employees with a combined 300 years of experience have left or been pushed out. "This has left the agency in free fall," she said. Goss also had some public missteps. In March 2005, just before Negroponte took over, he told an audience at the Ronald Reagan Presidential Library that he was overwhelmed by the duties of his job. "The jobs I'm being asked to do, the five hats that I wear, are too much for this mortal," Goss said. "I'm a little amazed at the workload." A number of former congressional colleagues released statements praising Goss on Friday, but not all were favorable. Senator Jay Rockefeller, the Intelligence Committee's top Democrat, said Goss' mission was to lead the agency with foresight, but his tenure was marked by a staff exodus and a demoralized work force. "The management problems hindering the effectiveness of our intelligence programs are not limited to the CIA however," Rockefeller said. "There are red flags throughout the intelligence community." Bush aides have been looking for ways to rescue his presidency from sagging poll ratings and difficulties with the Iraq war and his agenda in Congress. The shake-up began with the resignation of Andrew Card as chief of staff and his replacement by Joshua Bolten. Other changes have included the replacement of press secretary Scott McClellan with Fox News commentator Tony Snow. It wasn't immediately clear what's next for Goss, 67. He was supposed to retire after representing a Republican district on Florida's West Coast for 16 years, but he became CIA director when Bush called in 2004. Many former directors take consulting positions on various corporate boards. Goss and his wife own a central Virginia farm, where they raise cattle, sheep and chickens.