The commission investigating the attacks of Sept. 11, 2001, has a critical dual mission to fulfill -- to help our nation understand how the worst assault on our homeland since Pearl Harbor could have occurred and to outline reforms to prevent new acts of terrorism. Under the leadership of former governor Tom Kean and former congressman Lee Hamilton, the commission has acted with professionalism and skill. Its hearings and the reports it has released have been highly informative, if often disturbing. Sept. 11 united this country in shock and grief; the lessons from it must be learned in a spirit of unity, not of partisan rancor.

At last week's hearing, Attorney General John Ashcroft, facing criticism, asserted that "the single greatest structural cause for September 11 was the wall that segregated criminal investigators and intelligence agents" and that I built that wall through a March 1995 memo. This is simply not true.

First, I did not invent the "wall," which is not a wall but a set of procedures implementing a 1978 statute (the Foreign Intelligence Surveillance Act, or FISA) and federal court decisions interpreting it. In a nutshell, that law, as the courts read it, said intelligence investigators could conduct electronic surveillance in the United States against foreign targets under a more lenient standard than is required in ordinary criminal cases, but only if the "primary purpose" of the surveillance were foreign intelligence rather than a criminal prosecution.

Second, according to the FISA Court of Review, it was the justice departments under Presidents Ronald Reagan and George H.W. Bush in the 1980s that began to read the statute as limiting the department's ability to obtain FISA orders if it intended to bring a criminal prosecution. The practice of prohibiting prosecutors from directing intelligence investigations was first put in place in those years as well. Then, in July 1995, Attorney General Janet Reno issued written guidelines that spelled out the steps FBI intelligence agents and criminal investigators and prosecutors needed to follow when sharing information. The point was to preserve the ability of prosecutors to use information collected by intelligence agents.

Third, Mr. Ashcroft's own deputy attorney general, Larry Thompson, formally reaffirmed the 1995 guidelines in an Aug. 6, 2001, memo addressed to the FBI and the Justice Department. Ashcroft has charged that the guidelines hampered the department's ability to pursue terrorists Zacarias Moussaoui, Khalid al-Midhar and Nawaf al-Hazmi in August 2001, but his own department had endorsed those guidelines at the pivotal time.

Fourth, the memo I wrote in March 1995 -- which concerns information-sharing in two particular cases, including the original World Trade Center bombing -- permits freer coordination between intelligence and criminal investigators than was subsequently permitted by the 1995 guidelines or the 2001 Thompson memo. The purpose of my memo was to resolve a problem presented to me: facilitating investigations on both the intelligence side and criminal side at the same time. My memo directed agents on both sides to share information -- and, in particular, directed one agent to work on both the criminal and intelligence investigations -- to ensure the flow of information "over the wall." We set up special procedures because of the extraordinary circumstances and the necessity to prevent a court from throwing out any conviction in those cases. Had my memo been in place in August 2001 -- when, as Ashcroft said, FBI officials rejected a criminal warrant of Moussaoui because they feared "breaching the wall" -- it would have allowed those agents to obtain a criminal warrant without fear of jeopardizing an intelligence investigation.

Fifth, nothing in the 1995 guidelines prevented the sharing of information between criminal and intelligence investigators. Indeed, the guidelines require that FBI foreign intelligence agents share information with criminal investigators and prosecutors whenever they uncover facts suggesting that a crime has been or may be committed. The guidelines did set forth procedures, but those procedures implemented court decisions and, as noted, were reaffirmed by the Ashcroft Justice Department.

The Patriot Act, enacted after 9/11, together with an unprecedented appeal to the Foreign Intelligence Surveillance Court of Review, paved the way for the Justice Department to permit largely unrestricted information-sharing between intelligence and criminal investigators because the law changed the legal standard that had given rise to the guidelines in the first place. The Patriot Act says that electronic surveillance can be conducted in the United States against foreign threats as long as a "significant purpose" -- rather than the "primary purpose" -- is to obtain foreign intelligence.

This history has all been well-rehearsed in publicly available briefs, opinions and reports, all available to the 9/11 commission. I have -- consistent with the policy applied to all commissioners -- recused myself from any consideration of my actions or of the department while I was there. My fellow commissioners have spoken for themselves in rejecting the call by a few partisans that I step aside based upon false premises. I have worked hard to help the American public understand what happened on Sept. 11. I intend -- with my brethren on the commission -- to finish the job.

The writer is a member of the 9/11 commission and was deputy attorney general in the Clinton administration from March 1994 through March 1997.