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 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.

While a federal law, the Classified Information Protection Act, is intended to allow such trials to go forward without revealing secrets, in practice judges have not always agreed with the government that certain information can be withheld from a public trial. If it turns out that prosecutors miscalculated in predicting how a judge would rule on such evidentiary issues, the agency that had urged the Justice Department to bring the case might balk at letting it continue.

That said, there are possible situations in which it may be easier for prosecutors in the current cases to succeed — in particular, if they can locate e-mails in which a particular official disclosed a clearly sensitive secret.

Still, wide-ranging leak investigations can also have unintended consequences — as when Patrick J. Fitzgerald, the prosecutor investigating the disclosure during the Bush administration of the identity of a Central Intelligence Agency operative, Valerie Plame Wilson, ended up charging Vice President Dick Cheney’s top aide, I. Lewis Libby Jr., with lying to the F.B.I. under questioning.

In that investigation, Mr. Fitzgerald had been made a special counsel and delegated all the powers of the attorney general. Some Republicans, accusing the Obama administration of leaking information to make President Obama look tough, have called for special counsels to lead the new investigations, too.

Mr. Obama on Friday denied that his White House had sanctioned any leaking. And Mr. Holder rejected the need for a special counsel, instead appointing two United States attorneys without any special independence.

That could make a difference if the investigators want to subpoena reporters or their records, because Mr. Holder himself would have to sign off on such a request. Mr. Fitzgerald, by contrast, was able to subpoena a New York Times reporter, Judith Miller, on his own. (She ended up spending 85 days in jail after initially refusing to testify.)

Because leak investigations raise Constitutional issues about press freedoms, Justice Department regulations say that prosecutors may not subpoena reporters’ testimony or communications records unless they have exhausted all other means of getting the information they are seeking. Special counsels must obey such restrictions, too — and inevitable litigation over any subpoenas to the press mean courts will oversee their decisions.

“These cases are supposed to be difficult,” said Steve Aftergood, director of the Project on Government Secrecy with the Federation of American Scientists. “Investigators will have to balance several competing interests — including punishment of leaks, preservation of remaining secrets, and freedom of the press. That is a challenging assignment and it explains why there have been relatively few prosecutions over the years.” (Copyright 2012 The New York Times Company 06/10/12)