Lawmakers will examine two cases in which FBI staff uncovered signs of possible terrorist activity before the Sept. 11 attacks, but their warnings went unheeded.

The House and Senate intelligence committees will hold a hearing Tuesday into the FBI's handling of the arrest of Zacarias Moussaoui on immigration charges one month before the hijackings. Field agents had been denied permission to search his computer. Moussaoui has since been charged with conspiring in the attacks.

They will also look at the response to a July 2001 memo by Phoenix-based FBI agent Kenneth Williams suggesting that terrorists might be learning to fly commercial jets at a U.S. flight school. Williams was expected to testify Thursday, his appearance concealed by a screen, along with two other FBI agents, congressional officials said.

The committees are holding a joint inquiry into intelligence failures leading up to the attacks. At the inquiry's first public hearings last week, staff director Eleanor Hill presented two reports outlining missed counterterrorism opportunities before the attacks.

She also noted that some agents had seen potential dangers. They included an unnamed FBI agent in New York who, two weeks before the attacks, was prevented from pursuing two men who later turned out to be among the hijackers.

"There were individuals within the intelligence community who recognized the importance of what potentially was at stake and tried, though ultimately without success, to get organizations within the intelligence community to do the same," Hill said Friday.

Public hearings in the Sept. 11 attacks had been delayed for almost three months, partly because of questions about what information could be revealed in the Moussaoui case. Hill's report on the Moussaoui case was still being reviewed Monday by the Justice Department, and the part of Tuesday's hearing dealing with the case could be held behind closed doors.

U.S. intelligence officials on Monday disputed a charge in Hill's report that they "did not fully understand the importance of a key leader" in Al Qaeda. They said they have been pursuing this leader, Khalid Shaikh Mohammed, for years.

Officials said they provided some 600 documents to the inquiry's staff detailing their efforts to capture him. Mohammed has been associated with Al Qaeda since as early as 1995 and is on the FBI's 22 most-wanted terrorists list.

Mohammed worked with Ramzi Yousef, now in prison for the 1993 World Trade Center bombing, and two others in the Philippines on a number of plots, collectively termed "Project Bojinka." One plan including crashing a hijacked airplane into CIA headquarters outside of Washington. Of the four participants in Project Bojinka, only Mohammed remains free.

The four were linked to Al Qaeda through a financial operative named Khalifa, who is Usama bin Laden's brother-in-law, said U.S. officials speaking on the condition of anonymity. Khalifa is believed to be still at-large.

In January 1996, the U.S. government tried to have Mohammed detained in Qatar and turned over to U.S. authorities, but was unsuccessful, U.S. officials have said. By the middle of the year, the Qatari government reported it had lost track of him, sparking concerns that someone in the government had tipped him off.

There have been other attempts to catch him since then. The capture and interrogation of another senior Al Qaeda operative, Abu Zubaydah, provided information about Mohammed's role as a key planner of the Sept. 11 attack. In the last year, Mohammed has become the group's most active planner of new terrorist attacks, officials have said.