WASHINGTON (CNN) -- The CIA leak investigation is "not over," special prosecutor Patrick Fitzgerald said Friday after announcing charges against I. Lewis "Scooter" Libby, Vice President Dick Cheney's chief of staff.Fitzgerald said he will be keeping the investigation "open to consider other matters." But, he said, "the substantial bulk of the work in this investigation is concluded."Libby resigned Friday after a federal grand jury indicted him on five charges related to the leak probe: one count of obstruction of justice, two counts of perjury and two counts of making false statements. (Charges explained)Libby said in a written statement he is "confident that at the end of this process I will be completely and totally exonerated." "Today is a sad day for me and my family," he said."I have spent much of my career working on behalf of the American people," he said. "I have conducted my responsibilities honorably and truthfully." (Quote gallery)During an afternoon news conference, Fitzgerald outlined what he called the "very serious" charges. (See video of Fitzgerald outlining charges -- 13:50)"A CIA officer's name was blown, and there was a leak, and we needed to figure out how that happened, who did it, why, whether a crime was committed, whether we could prove it, whether we should prove it," he said. "Given national security was at stake, it was especially important that we find out accurate facts."The charges against a high-ranking official "show the world that this is a country that takes its law seriously," Fitzgerald said.Libby was charged with lying to FBI agents and to the grand jury about conversations with reporters. (Read the full text of the indictment)Libby testified that he heard CIA operative Valerie Plame's identity from Tim Russert of NBC News when, in fact, he learned of Plame's identify from other government officials, the indictment alleged."Mr. Libby's story that he was at the tail end of a chain of phone calls, passing on from one reporter what he heard from another, was not true," Fitzgerald said. "He was at the beginning of the chain of phone calls -- the first official to disclose this information outside the government to a reporter -- and then he lied about it afterwards, under oath and repeatedly," he said.Libby's attorney Joseph Tate said Fitzgerald concluded Libby did not violate a law that makes it a crime to intentionally disclose the identity of a covert agent. (Watch as news of Libby's resignation breaks -- :52) Tate said in a written statement that Libby is innocent, and he asked that the public not judge the case until a verdict is returned.He said he and his client were "dismayed" and "surprised" at the charges, and accused Fitzgerald of turning "alleged inconsistencies in Mr. Libby's recollection and those of others" into charges.White House 'saddened'Libby discussed Plame's identity in the summer of 2003 with reporters after her husband, diplomat Joseph Wilson, wrote a highly critical op-ed column in The New York Times that challenged intelligence used as part of the rationale for the U.S.-led war in Iraq.Descriptions of those conversations by reporters say Libby criticized the CIA and discussed Plame's identity in part to cast doubt on Wilson's accounts in the Times and elsewhere, the indictment alleged. The 22-page indictment said Libby -- before discussing Wilson and Plame with reporters -- discussed them with several people in the White House, identifying them only by title and alleging no wrongdoing on their part.They included Cheney, then-press secretary Ari Fleischer, an undersecretary of state identified by two sources as Marc Grossman and a senior White House official referred to as "Official A." Two other sources close to the probe said "Official A" is Karl Rove, President Bush's top political adviser. Senate Minority Leader Harry Reid, a Nevada Democrat, said the charges "suggest a senior White House aide put politics ahead of our national security and the rule of law."These are so far the only indictments in a nearly two-year investigation. If convicted on all counts, Libby, 55, could face a maximum of 30 years in prison and a $1.25 million fine, Fitzgerald said.Fitzgerald said Libby would not be arrested and refused to discuss any possible plea deal. Court officials said Judge Reggie B. Walton has been assigned to the case.The indictment comes at a time of political difficulty for the White House and Republicans, with the president's approval ratings at a low ebb. Bush on Friday called the legal proceedings "serious" and said he accepted the resignation of Libby, who also was an assistant to the president and a national security adviser to Cheney. (Watch Bush's reaction -- 1:08)Speaking to reporters at the White House, the president said Libby has "sacrificed much in his service to this country," and he cautioned that "in our system each individual is presumed innocent and entitled to due process.""While we're all saddened by today's news, we remain wholly focused on the many issues and opportunities facing this country," he said. Cheney said in a statement that he accepted Libby's resignation "with deep regret," calling him "one of the most capable and talented individuals I have ever known."'No decision'Rove, who testified four times before the grand jury, was not indicted Friday, but sources said he is not out of legal jeopardy. (Full story)Rove's attorney Robert Luskin issued a statement Friday in which he said Fitzgerald "advised Mr. Rove that he has made no decision about whether or not to bring charges.""We are confident that when the special counsel finishes his work, he will conclude that Mr. Rove has done nothing wrong," Luskin's statement said.The event that triggered the legal and political quagmire was a syndicated newspaper column by Robert Novak, published on July 14, 2003, about Wilson.Before Novak's column, Plame's role as a CIA officer was classified and "not widely known" outside the intelligence community, Fitzgerald said. In a written statement, Wilson on Friday called the indictments an "important step in the criminal justice process.""It is certainly not a day to celebrate," Wilson said, adding that he and his wife "are confident that justice will be done." "Revealing my wife Valerie's secret CIA identity was very wrong and harmful to our nation," Wilson said. "I feel that my family was attacked for my speaking the truth about the events that led our country to war."CNN's Kelli Arena, Dana Bash, John King and Kevin Bohn contributed to this report.