It was in response to Libby's March 17 Third Motion to Compel Discovery (pdf). Here is a complete list (pdf) of the documents Libby is requesting. Libby raises the issue of his grand jury testimony and the disclosure of the NIE report in the motion and seeks documents related to both. (A prior detailed discussion of the motion is here.) His request partially stems from a letter Fitz sent on January 23, 2006:

In a letter dated January 23, 2006, the government informed the defense that it intends to introduce all of Mr. Libby's grand jury testimony at trial. (Jan. 23, 2006 Ltr. from Patrick J. Fitzgerald to William Jeffress, et al., at 6, attached as Exhibit A.) The two transcripts collectively total 389 pages. The government also informed us that it seeks to make an issue at trial of Mr. Libby's alleged disclosures of a portion of the content of the October 2002 National Intelligence Estimate on Iraq's Continuing Programs for Weapons of Mass Destruction, known as the NIE,1 a subject the government claims is "inextricably intertwined with the narrative of spring 2003." (Id.)

Because Mr. Libby's testimony spanned a number of subject matters and because we have no way of knowing which of those matters, in addition to the NIE, the government will raise at trial, we are compelled to seek discovery regarding certain of the matters raised in the questions or answers given during Mr. Libby's testimony.

So Fitz responded with details of Libby's grand jury testimony that the Government thinks will be material at trial. Fitz then responds to a wide range of other documents Libby is requesting, particularly those originating with agencies outside the Office of the Vice President, which they agree Fitz has turned over (with the exceptions of the ones recently acquired which Fitz says they are in the process of locating and will turn over.)

In another portion of the motion, Libby requests statements of potential witnesses, reports and

All documents or information concerning the identity of any government official outside the CIA who was aware prior to July 14, 2003 that Ms. Wilson worked for the CIA.

Here is some of Libby's argument for disclosure of the documents from the March 17 request:

The indictment relies heavily on the testimony of at least seven government officials about conversations with Mr. Libby that allegedly occurred between May 29 and July 8, 2003. The government claims that Mr. and/or Ms. Wilson were referred to in all of these conversations, and that these discussions should have been memorable to Mr. Libby three to nine months later. When these witnesses testify at trial, issues of memory, credibility, veracity and even potential bias will arise. It is inconceivable to think that the defense could adequately prepare to cross-examine these critical government witnesses without access to relevant memoranda, emails, and other documents from their files and the files of their employers.

....The prosecution has an interest in continuing to overstate the significance of Ms. Wilson's affiliation with the CIA. Doing so makes it easier to suggest that Mr. Libby would not have forgotten or confused his conversations concerning Ms. Wilson and has therefore intentionally lied. In contrast, the defense intends to present a more complete and accurate narrative. The defense will show that during the controversy about the "sixteen words" in the President's 2003 State of the Union address and about Ambassador Wilson's criticism of the Bush Administration, government officials, including Mr. Libby, viewed Ms. Wilson's identity as at most a peripheral issue. To the extent that these officials were focused on Mr. Wilson, they were concerned with publicly disputing mistaken or misleading reports about his trip and his findings, not with where his wife worked.

The prosecution has largely denied Mr. Libby's requests for documents from agencies other than the Office of the Vice President ("OVP"). The Administration's response to Mr. Wilson's criticism and the "leak" of his wife's name, however, cannot be fully understood by focusing on the OVP alone. The actions of government officials from the White House, the State Department and the CIA - and the documents they generated - are part and parcel of this story. Indeed, many of the government's likely witnesses are or were high ranking employees at the White House, the State Department and the CIA.

In addition, the events alleged in the indictment occurred during a period of increasing bureaucratic infighting, when certain officials at the CIA, the White House, and the State Department each sought to avoid or assign blame for intelligence failures relating to Iraq's weapons of mass destruction ("WMD") capability. The fingerpointing that went on within the Executive Branch about who was to blame will be a key issue in examining many of the government's witnesses because it goes to the question of bias.

When the full context of the controversy over the sixteen words is presented to a jury, the jury will see that the relevant events were far more complex than the government has suggested in its discovery responses, and that the role of Ms. Wilson was peripheral. If the jury learns this background information, and also understands Mr. Libby's additional focus on urgent national security matters, the jury will more easily appreciate how Mr. Libby may have forgotten or is remembered the snippets of conversation the government alleges were so memorable.

The documents requested by this motion are also highly relevant to questions of motive. The defense has the right to make an affirmative showing that Mr. Libby had no motive to lie to the FBI or to the grand jury. Documents from other agencies, especially the White House and the State Department, will help the defense show that the Administration did not launch a concerted effort to "punish" Mr. Wilson by leaking his wife's identity, as has been suggested by a number of potential witnesses, including a key government witness, Time magazine reporter Matthew Cooper. These documents will also corroborate the defense position that neither Mr. Libby nor anyone with whom he worked closely had done anything wrong with respect to Mr. Wilson or his wife.

It seems to me that Fitz' filing was directly responsive to the discovery requests made by Libby. Since Fitz is opposing most of Libby's requests, particularly documents from agencies other than the Office of the Vice President, it is incumbent on him to tell the court the basis for his opposition.

In related news, the Los Angeles Times today has a good article on how Fitz's latest filing may hurt Libby's defense. It also has quotes from those supportive of Libby's position.