Sen. Richard Shelby (search), R-Ala., has for months been the focus of an investigation into the leak of classified intelligence intercepts linked to the Sept. 11 terror attacks, according to current and former law enforcement officials.

Shelby, who chaired the Senate Intelligence Committee (search) at the time of the attacks, denied that he or his staff knowingly leaked classified information and said he has no knowledge about where the 18-month-old investigation would head.

"At no time during my career as a United States senator and, more particularly, at no time during my service as chairman of the Senate Select Committee on Intelligence have I ever knowingly compromised classified information," Shelby said in a statement released this week.

Two law enforcement officials and one former official, all speaking on condition of anonymity, said a grand jury in Washington has heard testimony about the disclosure in 2002 of two messages intercepted by the National Security Agency (search) a day before the Sept. 11, 2001, terror attacks.

Those messages contained the words "the match begins tomorrow" and "tomorrow is zero day." The intercepts were not translated from Arabic until Sept. 12.

The intercepts had been disclosed by the NSA director, Lt. Gen. Michael Hayden, during a private meeting of a joint House-Senate intelligence committee that investigated the Sept. 11 attacks. Shelby was on the panel at the time.

House Minority Leader Nancy Pelosi, D-Calif., who served with Shelby on the intelligence panel, said Thursday she and the Alabama senator had been privy to more secrets than others on the committee because of their senior status.

"I know of no instance when any of that information was disclosed publicly," Pelosi said. "So my experience is a positive one as far as Sen. Shelby is concerned."

In his statement, Shelby said he had no contact with leak investigators "for well over a year," and he and his staff had cooperated fully. He did not describe the nature of that cooperation.

Shelby spokeswoman Virginia Davis said Friday the senator had no information indicating he was the focus or target of the probe. The law enforcement officials nonetheless said the probe has centered on him.

The investigation involves the FBI, CIA and NSA and is incomplete, the officials said. No decision has been made whether anyone will be prosecuted for knowingly leaking the classified material, which would be a felony if proved.