A veteran FBI agent has been arrested and accused of being a double agent.
The White House says the allegations are disturbing and security chiefs say the agent has caused considerable damage to US security.
Father-of-six Robert Philip Hanssen, 56, is only the third FBI agent to be accused of spying.
He was arrested at his home outside Washington after agents saw him deposit a package of classified information at a "dead drop" in a Virginia park.
He is to be arraigned in a court in Alexandria, Virginia.
Underscoring the gravity of the case, former FBI Director William Webster will lead a panel that will assess the impact of the alleged espionage.
White House spokesman Ari Fleischer said: "The president is troubled by the allegations. They are disturbing."
He says President Bush had been told of the case before Hanssen was arrested.
Hanssen was assigned to FBI headquarters in Washington, and had been an FBI agent for 27 years.
He spent most of his career in counterintelligence, spying on Russian government outposts in the US . He had been under surveillance for about four months.
Among secrets disclosed by Hanssen included US methods for conducting electronic surveillance. He also may have confirmed information originally supplied to them by convicted CIA spy Aldrich Ames for the Russians.