For U.S. Inquiries on Leaks, a Difficult Road to Prosecution WASHINGTON - Anger over leaks of government secrets and calls for prosecution have once again engulfed the nation's capital. Under bipartisan pressure for a crackdown, Attorney General Eric H. Holder Jr. on Friday announced the appointment of two top prosecutors to lead investigations into recent disclosures. But the prospects for those efforts are murky. Historically, the vast majority of leak-related investigations have turned up nothing conclusive, and several of the nine that have been prosecuted - six already under the Obama administration, and just three more under all previous presidents - collapsed. "These cases are very difficult to pursue," said Kenneth L. Wainstein, a former assistant attorney general for national security under President George W. Bush. Still, the Obama administration is facing intense pressure to identify and make examples of any officials who helped bring to light a series of recent disclosures - including new information about the Obama administration's drone strikes, a joint effort by the United States and Israel to damage Iranian nuclear equipment with a computer virus, and the foiling of a terrorist plot with help from a double agent. The reports appeared in several recent books and articles, including some by The New York Times. Many people are surprised to learn that there is no law against disclosing classified information, in and of itself. The classification system was established for the executive branch by presidential order, not by statute, to control access to information and how it must be handled. While officials who break those rules may be admonished or fired, the system covers far more information than it is a crime to leak. Instead, leak prosecutions rely on a 1917 espionage statute whose principal provision makes it a crime to disclose, to persons not authorized to receive it, national defense information with knowledge that its dissemination could harm the United States or help a foreign power. To win such a case at trial, prosecutors have to prove to a jury that the leaked information met that standard, including showing why its disclosure was harmful. To date, there has been only one successful trial of an accused leaker - an intelligence analyst who gave satellite pictures of a Soviet shipbuilding facility to Jane's Defense Weekly in 1984. Several defendants in other leak cases pleaded guilty, avoiding a fight over whether the information they had passed on qualified. Other cases were dismissed. Several lawmakers last week proposed updating and strengthening secrecy laws, reviving proposals that have periodically been made after other disclosures. Most of those episodes faded away without action, but in 2000 Congress passed a bill that would have made the disclosure of any classified information a felony. President Bill Clinton vetoed it. In 2002, President George W. Bush's attorney general, John Ashcroft, told Congress that no new laws were necessary. Identifying a leaker is also rarely easy, since there are often dozens or hundreds of officials who had access to the information. But it is easier today than in earlier eras to build a circumstantial case that a particular official talked to a reporter because modern communications technology - like e-mail - leaves trails. Several of the recent disclosures, however, resulted from deeply reported projects. Such articles tend to have diffuse sourcing, making it hard to isolate who first disclosed the essence of what later becomes an article. On those rare occasions when there is an identifiable leaker, the government must still decide whether prosecuting him would mean divulging too many secrets to be worth it - starting, usually, with having to confirm in public that a particular leak was accurate. Defendants who choose to fight often rely on a so-called graymail defense. This involves making the disclosure of further classified information a centerpiece of their right to a fair trial by pushing for even more revelations, such as identifying other people at the agency who had access to the same knowledge.