by Colin Moynihan

When the scheduled federal trial begins this month for two Texas men who were arrested during the Republican National Convention on charges of making and possessing Molotov cocktails, one of the major witnesses against them will be a community activist who acted as a government informant.

Brandon Darby, an organizer from Austin, Tex., made the news public himself, announcing in an open letter posted on Dec. 30 [1] on Indymedia.org that he had worked as an informant, most recently at last year's Republican convention in St. Paul.

"The simple truth is that I have chosen to work with the Federal Bureau of Investigation," wrote Mr. Darby, who gained prominence as a member of Common Ground Relief [2], a group that helped victims of Hurricane Katrina in New Orleans.

He added, "I strongly stand behind my choices in this matter."

Mr. Darby's revelations caused shock and indignation in the activist community, with people in various groups and causes accusing him of betrayal.

"The emerging truth about Darby's malicious involvement in our communities is heart-breaking and utterly ground-shattering," said the Austin Informant Working Group, a collection of activists from the city who worked with Mr. Darby. "Through the history of our struggles for a better world, infiltrators and informants have acted as tools for the forces of misery in disrupting and derailing our movements."

Mr. Darby's letter answered lingering questions in the case of the two Texas men, David McKay and Bradley Crowder, both also from Austin. They are scheduled to go on trial in Minnesota on Jan. 26, and if convicted on all counts, each faces a prison sentence of up to 30 years.

Neither the United States attorney's office in Minnesota nor the F.B.I. would comment on Mr. Darby's announcement.

"As a matter of policy, we're not going to confirm or deny the identity of anybody who gives us information confidentially," said E. K. Wilson, an F.B.I. spokesman in Minnesota.

But in a telephone interview, Mr. Darby said that he had provided information leading to the arrest of Mr. Crowder and Mr. McKay, and that he planned to testify at their trial.

Mr. Darby would not provide details about his undercover activities, but said he had also worked as an informant in cases not involving the convention. He defended his decision to work with the F.B.I. as "a good moral way to use my time," saying he wanted to prevent violence during the convention at the Xcel Energy Center.

Documents that activists said were given to defense lawyers by the prosecution and printed on F.B.I. letterhead indicated that an informant - now identified as Mr. Darby - carried out a thorough surveillance operation that dated back to at least 18 months before the Republican gathering. He first met Mr. Crowder and Mr. McKay in Austin six months before the convention.

Mr. Darby provided descriptions of meetings with the defendants and dozens of other people in Austin, Minneapolis and St. Paul. He wore recording devices at times, including a transmitter embedded in his belt during the convention. He also went to Minnesota with Mr. Crowder four months before the Republican gathering and gave detailed narratives to law enforcement authorities of several meetings they had with activists from New York, San Francisco, Montana and other places.

One of his last conversations with Mr. McKay ended in an alley in Minneapolis, according to court documents, with Mr. Darby recording Mr. McKay talking about plans to use Molotov cocktails.

The F.B.I. reports mentioned dozens of people, most of whom have not been accused of any crime. In addition to listing biographical and physical particulars, Mr. Darby frequently offered observations on the motives, attitudes and states of mind of activists with whom he dealt.

"Part of what intrigues me is not only how he operates but what is the role of the F.B.I. in how he operates," said Lisa Fithian, an organizer who is named in the reports. "We don't know what we're dealing with here."

Some former friends of Mr. Darby have denounced him as a provocateur and said he might have enabled or encouraged Mr. Crowder and Mr. McKay to break the law. Mr. Darby denied that.

An F.B.I. agent swore in an affidavit that at one point Mr. McKay acknowledged that he intended to use firebombs. Such devices were never used, and both defendants have pleaded not guilty.

"The claim that the case is solely based on the testimony of informants is simply a wanton and willful untruth," Mr. Darby said in the interview. "It omits the physical evidence, the confession and possibly the testimony of many others."

In 2005, Mr. Darby went to New Orleans after Hurricane Katrina struck, joining Common Ground Relief as it provided medical attention and helped repair homes. He became a visible member of the group, sometimes acting as a spokesman and appearing on "The Tavis Smiley Show" on PBS.

When The St. Paul Pioneer Press published an article in October that cited an unidentified source who named Mr. Darby as an informant in the case against Mr. Crowder and Mr. McKay, a co-founder of Common Ground, Scott Crow, defended Mr. Darby publicly and warned against "rumors, conjecture and innuendo."

"I put it all on the line to defend him when accusations first came out," Mr. Crow said. "Brandon Darby is somebody I had entrusted with my life in New Orleans, and now I feel endangered by him."

Mr. Darby acknowledged that many people he spied on might not accept his explanation that he was motivated by conscience.

"I am well aware," he said, "that I've stepped outside of accepted behaviors and that I've committed a sin in the eyes of many activists."