The New York Times reports the investigation is looking at whether people have lied to federal investigators or mishandled classified documents.
The newspaper also reports the special prosecutor leading the probe, Patrick Fitzgerald, may be ready for another round of grand jury testimony, and perhaps close to ending his investigation. It is not clear whether he has decided to press charges or not.
The case concerns the publication last summer of the name of a CIA agent who is married for former Ambassador Joseph Wilson. The agent's name emerged after Wilson publicly criticized the Bush administration for making a false allegation against Iraq.
Naming a clandestine agent could violate federal law. The CIA asked for an investigation of the leak, and the Justice Department agreed. Three months ago, Attorney General John Ashcroft recused himself from the case, and Fitzgerald took over.
Several top White House staff members have already testified before a grand jury in the case, and still more have met with the FBI.
The Times reports Fitzgerald is looking at possible contradictions between their statements. Documents like e-mails and phone records may also clash with the statements.
In addition, investigators want to know if the CIA agent's name was taken from a classified document. It could be illegal to disclose any information from such documents.
The Times reports Republicans are worried that if Fitzgerald pursues charges unrelated to the leak itself — like someone allegedly lying to the grand jury — the probe will drag on like the Whitewater probe of the Clinton administration, which began as an investigation of real estate deals, but ultimately examined President Clinton's lying about sexual improprieties.
The newspaper also reports that Fitzgerald, if he decides not press charges, may face pressure from Democrats to reveal the reasons behind his decision. But law bars him from disclosing what a grand jury heard, and prosecutors usually do not discuss an investigation where no charges are brought. Fitzgerald might ask a judge for permission to issue a report on the matter, The Times reports.
Wilson, who was the U.S. charge d'affaires in Baghdad at the onset of the 1991 Persian Gulf War, was dispatched to Niger in early 2002 to investigate a report that Iraq had tried to purchase yellow-cake uranium, which could be used to make a nuclear bomb.
He reported back that he saw no evidence of an Iraqi bid and doubted it could have succeeded.
The CIA prevented the Bush administration from mentioning the Niger allegation in two speeches in the fall of 2002. The agency tried, but failed, to prevent Britain from including the Niger charge in a dossier laying out the case for war.
In his January 2003 State of the Union speech, President Bush referred to the British report of the Niger allegation.
Days later, the International Atomic Energy Agency said documents purporting to be evidence of the Niger bid were forgeries. An FBI investigation is trying to learn who forged them.
The allegation that Iraq possessed weapons of mass destruction was central to the White House rationale for invading Iraq in March 2003.
Wilson went public with his findings in July 2003, prompting the White House to retract the Niger claim.
Administration officials said it was only one aspect of the case for war. But no weapons of mass destruction, or active programs to produce nuclear, chemical or biological weapons, have been uncovered in months of searches in Iraq.
Newsday reported last month that the grand jury has subpoenaed the records of phone calls from Air Force One, the president's plane, made the week before the name of the officer was published.
The three subpoenas to President Bush's Executive Office also seek the July records created by an internal task force called the White House Iraq Group, which was created to publicize the threat of Saddam Hussein, Newsday said. The newspaper cited documents that it obtained.
White House staff records also were sought regarding contacts with several other news organizations including The Associated Press, The Washington Post, Time magazine, NBC, The New York Times and The Wall Street Journal, Newsday said. There has been no record of journalists being subpoenaed.
At least four current and former officials involved in the White House communications operation have testified before the grand jury, according to people familiar with the probe.
Press secretary Scott McClellan and Adam Levine, who formerly worked in the press office, testified before the panel in February. A deputy in McClellan's office, Claire Buchan, said she appeared before the panel on Jan. 30.
Mary Matalin, a Republican consultant who headed Vice President Dick Cheney's communications office until December, confirmed that she testified on Jan. 21.
A number of other current or former officials have been interviewed by the FBI, including top Bush political adviser Karl Rove, communications chief Dan Bartlett, former spokesman Ari Fleischer and Cheney spokeswoman Cathie Martin, said two sources who spoke on condition of anonymity.
The Post reports the FBI has also talked to Cheney chief of staff I. Lewis "Scooter" Libby, along with Levine, Matalin and McClellan.
The Post reports the FBI has conducted aggressive questioning covering White House emails that were turned over to the investigators, cell phone calls, phone logs and even specific conversations with reporters.
Investigators earlier tried to encourage journalists to talk by asking White House staff to sign waivers freeing reporters of any promises of confidentiality. But according to The Post, most officials were advised by the lawyers not to sign, and did not.