Four current and former White House officials, all part of the West Wing's press operations, are now known to have testified before a grand jury in the investigation of a leak of the name of an undercover CIA officer.

Federal investigators are looking into whether anyone at the White House disclosed to columnist Robert Novak (search) and other members of the media the name of Valerie Plame (search), wife of former Ambassador  Joseph C. Wilson (search), who was involved last year in a dispute with the White House over pre-war Iraq intelligence.

Press Secretary Scott McClellan is one of the aides to have appeared before the grand jury.

"I obviously want to do my part to cooperate, and if there's something that can help those that are leading this investigation get to the bottom of it, I am more than happy to share that information with them," McClellan told reporters on Tuesday.

Another one of the officials known to have been called before the grand jury is Mary Matalin (search), former communications chief for the office of Vice President Dick Cheney. Adam Levine, who formerly worked in the press office, and McClellan deputy Claire Buchan have also testified.

Several other officials are known to have testified, though it is not known how many. The names were chosen by the grand jury based on e-mails handed over by the White House that included the aides' names.

Many other White House aides, including senior adviser Karl Rove (search), communications chief Dan Bartlett, former spokesman Ari Fleischer, Cheney spokeswoman Cathie Martin and Cheney chief of staff Lewis Libby (search), have all been interviewed by the FBI.

One person close to the investigation said that Levine may have been questioned because Fleischer and Bartlett were with Bush on a July 7-12 trip to Africa just prior to publication of Novak's column, and McClellan, then Fleischer's deputy, was on vacation.

Few of the aides are acknowledging they have been called.

"There is an ongoing investigation right now. We want to be as helpful as we can to those that are leading that investigation," McClellan said, who added that President Bush ordered all staff to cooperate fully with the investigation.

Investigators are probing whether any White House officials deliberately leaked Plame's name in order to discredit Wilson, who had argued the Bush administration exaggerated intelligence about Iraq's nuclear weapons program in order to build a case for war.

Wilson had prepared a report for the CIA on Iraqi efforts to buy uranium in Africa, which undercut a 16-word statement in the President's 2003 State of the Union address.

Anonymous officials irritated at Wilson suggested he'd gotten the mission only because his wife worked for the CIA, a suggestion first reported by Novak last July.

"His wife Valerie Plame, is an agency operative on weapons of mass destruction. Two senior administration officials told me Wilson's wife suggested sending him to Niger," Novak wrote.

Wilson now is working as a foreign policy adviser to Democratic presidential candidate John Kerry and has made several campaign appearances for the Massachusetts senator.

Senior Democrats on the House Judiciary and Government Reform committees questioned in a letter to Bush on Tuesday whether White House aides were being as forthcoming as they should be. They said most staffers have refused on the advice of counsel to sign Justice Department waivers aimed at releasing journalists from promises to protect confidential sources.

"Simply put, individuals who are unwilling to assist in discovering who leaked Valerie Plame's identity should not be trusted with our nation's most sensitive state secrets," said Rep. John Conyers, D-Mich., and Henry Waxman, D-Calif.

Under the Intelligence Identities Protection Act (search), the penalty for exposing an undercover agent is a violation of the law, punishable by up to 10 years in prison and $50,000 in fines.

However, prosecuting someone for telling Novak about Plame's employment may be difficult. Knowingly exposing an undercover agent is a violation of the law, but only if the leaker knows that the agent is working undercover. Some non-political officials say, few, if any officials, would have known Plame's role, and whoever did talk to Novak probably did not know.

In fact, the source said, even in the intelligence community few people knew.

The person who gave up the information would have had to have received the information in his or her official capacity, got the information from someone with official clearance and did so in defiance of agency efforts to keep the employee's name a secret, said Joseph DiGenova, former U.S. Attorney for the District of Columbia, now in private practice.

DiGenova added that for an individual to overhear the name of an agent and pass it on is not enough for prosecution.

"A lot of people knew that [Plame] worked for the CIA," DiGenova told Fox News. "People outside of the government knew that she worked at the agency. They did not know probably, that she worked in WMD — weapons of mass destruction — and was doing undercover work. But in order for it to be a crime, you must know that is what she did."

Prosecutors are asking those who appear before the grand jury to sign a waiver releasing reporters from the obligation to protect the individual as a source. Some have signed, others have not.

But one knowledgeable person said prosecutors will eventually have to go after journalists to get the truth and that would lead to nasty legal wrangles that could delay the case for a year or more. 

"Whether you want to start locking up journalists before you know whether or not a crime has even been committed seems to be a pretty bad exercise of judgment," DiGenova said.

Investigators have not yet decided whether to try to interview or subpoena Novak or other reporters, said sources familiar with the case. Such attempts usually meet with resistance from news organizations, who say protection of confidential sources is crucial to newsgathering.

Under Justice Department guidelines, prosecutors must exhaust all other means of learning information in a criminal case before attempting to extract it from reporters. Criminal law experts say there is no blanket immunity for news media personnel in such investigations.

"Any time you're dealing with a press source, you have certain hurdles that would not exist with a normal citizen," said Michael O'Neill, law professor at George Mason University in Fairfax, Va. "By and large, especially when you're dealing with a criminal investigation, it doesn't make any difference and the press would have to give up that information."

Fox News' Jim Angle and The Associated Press contributed to this report.