President Bush (search), under pressure from Democrats to name a special prosecutor, said Monday he is confident the Justice Department (search) can thoroughly investigate the leak of an undercover CIA officer's identity.

"We will cooperate fully with the Justice Department. I have every confidence in the world that the Justice Department will do a good, thorough job," Bush said during a joint news conference at the White House with Mwai Kibaki (search), president of Kenya.

Bush, who called the leak a "criminal matter," said he did not know who leaked the information. "You hear all kinds of rumors," he said. Leading Democrats have called for a special prosecutor, saying the administration cannot investigate itself.

White House spokesman Scott McClellan said if the leaker is identified, he or she will be dismissed.

"If anyone in this administration was responsible for the leaking of classified information, they would no longer be working in this administration," he said.

About 200 of the 2,000 White House employees ordered to turn over relevant documents to Justice Department investigators had done so by Monday morning. Many of those were staff members who simply certified they had no documents that might help the Justice Department investigation, McClellan said.

In the East Room news conference, Bush said he expected aides to turn over information in a timely fashion. "I want there to be full participation because ... I am most interested in finding out the truth," he said.

All White House staffers have until 5 p.m. EDT Tuesday to certify either that they have produced relevant documents or have no such documents. The order covers materials such as electronic records, telephone logs, correspondence, computer records, notes and calendar entries.

Investigators are trying to determine who leaked to the three journalists the identity of Valerie Plame, a CIA operations officer who has served overseas. She is married to former Ambassador Joseph Wilson, who publicly accused the Bush administration of manipulating intelligence to exaggerate the threat from Iraq.

"This is something that the president wants to get to the bottom of as quickly as possible, because this is a very serious matter that needs to be taken seriously, when you're talking about the leak of classified information," McClellan said.