WASHINGTON -- Even before the FBI conducted 550 interviews of officials and seized the phone records of Associated Press reporters in a leak investigation connected to a 2012 article about a Yemen bomb plot, agents had sought the same reporters' sources for two other articles about terrorism.
In a separate case last year, FBI agents asked the White House, the Defense Department and intelligence agencies for phone and email logs showing exchanges with a New York Times reporter writing about computer attacks on Iran. Agents grilled officials about their contacts with him, two people familiar with the investigation said.
And agents tracing the leak of a highly classified CIA report on North Korea to a Fox News reporter pulled electronic archives showing which officials had gained access to the report and had contact with the reporter on the day of the leak. They studied one official's entrances and exits from the State Department, obtained his Yahoo email information and even searched his hard drive for deleted files, documents unsealed this month showed.
The emerging details of these and other cases show just how wide a net the Obama administration has cast in its investigations into disclosures of government secrets, querying hundreds of officials across the federal government and even some of their foreign counterparts.
The result has been an unprecedented six prosecutions and many more inquiries using aggressive legal and technical tactics. A vast majority of those questioned were cleared of any leaking.
On Thursday, President Barack Obama ordered a review of the Justice Department's procedures for leak investigations involving reporters, saying he was concerned that such inquiries chilled journalists' ability to hold the government accountable. Yet he made no apology for the scrutiny of the many officials whose records were searched or who had been questioned by the FBI.
"He makes the case that we have 18-year-olds out fighting wars and acting like adults, and we have senior administration officials quoted in stories acting like children," said Tommy Vietor, a former National Security Council spokesman. Mr. Obama and top administration officials say that some of the leaks have endangered Americans, disrupted intelligence operations and strained alliances with other countries.
Some officials are now declining to take calls from certain reporters, concerned that any contact may lead to investigation.
Some complain of being taken from their offices to endure uncomfortable questioning. And the government officials typically must pay for lawyers themselves, unlike reporters for large news organizations whose companies provide legal representation.
"For every reporter that is dealing with this, there are hundreds of national security officials who feel under siege -- without benefit of a corporate legal department or a media megaphone for support," a former Obama administration official said. "There are lots of people in the government spending lots of money on legal fees."
When an agency spots classified information in the news media, officials file what is called a "Crimes Report" with the Department of Justice, answering 11 standard questions about the leak, including the effect of the disclosure "on the national defense."
FBI agents then set out to find the leaker, a process that has become far easier in recent years as email and other electronic records have proliferated.
Officials who have been questioned in the current investigations often are reluctant to describe their experiences.
One of the most striking recent revelations about the Obama administration's pursuit of leakers was the disclosure that the Justice Department had obtained emails from the Google account of James Rosen of Fox News, in which he corresponded with a State Department analyst suspected of leaking classified information about North Korea. Investigators routinely search the emails of suspected leakers, but Congress has forbidden search warrants for journalists' work product materials unless the reporter has committed a crime.
A 2010 affidavit seeking the warrant -- necessary, an FBI agent wrote, because the analyst had deleted emails in his own accounts -- declared that Mr. Rosen qualified for that exception because he violated the Espionage Act by seeking secrets to report.
No U.S. journalist has been prosecuted for publishing classified information, and the administration insisted that it has no intention of doing so. Attorney General Eric Holder signed off on the warrant request, the department has acknowledged.
Yet the government's willingness to go after journalists' email and phone records without warning their news organizations -- a practice that allows them to challenge the demand in court -- appears to be increasing.
"There seems to have been a shift in attitude," said Steven Aftergood, who directs a project on government secrecy for the Federation of American Scientists. "The latest revelations indicate that reporters' communications are now fair game."
It is not clear how often the government has obtained communications records of reporters.
While Fox News was informed nearly three years ago about the subpoena for call logs for five lines related to Mr. Rosen -- apparently after the phone company had already provided them -- it did not publicly disclose the action. Instead, it emerged only this month when court papers were unsealed that also showed that the government had separately obtained a warrant for the contents of Mr. Rosen's private email account. A lawyer and spokesmen for Fox did not respond to requests for comment.
Erin Madigan, an AP spokeswoman, also said the AP was not contacted by the government about leak investigations into two 2010 articles, although it was aware of published reports that there were inquiries under way.