A special counsel is focusing on whether Vice President Dick Cheney played a role in leaking a covert CIA agent's name, according to people familiar with the probe that already threatens top White House aides Karl Rove and Lewis Libby.

The special counsel, Patrick Fitzgerald, has questioned current and former officials of President George W. Bush's administration about whether Cheney was involved in an effort to discredit the agent's husband, Iraq war critic and former U.S. diplomat Joseph Wilson, according to the people.

Fitzgerald has questioned Cheney's communications adviser Catherine Martin and former spokeswoman Jennifer Millerwise and ex-White House aide Jim Wilkinson about the vice president's knowledge of the anti-Wilson campaign and his dealings on it with Libby, his chief of staff, the people said. The information came from multiple sources, who requested anonymity because of the secrecy and political sensitivity of the investigation.