A congressional committee probing ties between FBI agents and mob informants adjourned abruptly Friday morning after University of Massachusetts President William M. Bulger refused to testify about his fugitive brother.

Bulger, who had testified to a grand jury last year under a grant of immunity, invoked his Fifth Amendment right not to answer questions. His attorney had tried unsuccessfully to have the hearing postponed or held behind closed doors.

The House Committee on Government Reform has been investigating the improper relationship between some Boston FBI agents and their mob informants.

The panel wanted to question Bulger about older brother James "Whitey" Bulger, a notorious gang leader who is wanted in connection with 21 murders. Whitey Bulger, who has been on the run for nearly eight years, was also a top-echelon informant who provided the FBI with information about the New England faction of the Italian Mafia.

Committee Chairman Dan Burton, after denying the request to delay the hearing or make it closed, asked if Bulger had talked to his brother since 1995, where his brother was then, and where he is now.

Bulger, in response, said he was invoking his right against self-incrimination.

"The Fifth Amendment's basic function is to protect innocent men who might be ensnared by ambiguous circumstances," he said. "I find myself in such circumstances."

Burton then asked if he intended to answer all questions in the same manner.

"Yes, sir," said Bulger, who was one of the state's most colorful and powerful politicians during 17 years as Senate president.

Whitey Bulger went on the run in January 1995 after being tipped off by former FBI agent John J. Connolly Jr. that he was about to be indicted. Connolly was convicted earlier this year for warning him and other mobsters about their indictments.

Excerpts from William Bulger's grand jury testimony were published in The Boston Globe earlier this week. A judge said Thursday that the immunity from prosecution he was granted for that appearance would not extend to his appearance before the committee.

In his testimony, Bulger acknowledged he had received a phone call from his brother several weeks after he became a fugitive. He said he didn't urge his brother to surrender, "because I don't think it would be in his interest to do so."

Bulger's lawyer, Thomas R. Kiley, said before the hearing that it was unfair that Bulger would not be given the opportunity to review his 2001 grand jury testimony.

"Any mistakes that he might make, could be used against him," Kiley said. "If they cannot have one Bulger, we fear they will have another." Kiley also spoke of Bulger's "ache" for what his brother had become.

After the hearing adjourned, Burton said he was disappointed that the committee was not able to get answers from Bulger.

"I understand that Whitey Bulger is his brother, but he's one of the 10 most wanted fugitives in the United States," Burton said.

At Thursday's session of the committee's hearing, a former federal prosecutor, Jeremiah O'Sullivan, testified he knew two mobsters were murderers and FBI informants, but did not indict them in a horse race fixing case because he felt he did not have enough evidence to convict them.

O'Sullivan also acknowledged that Connolly and another agent asked him not to indict Whitey Bulger and another man, Stephen "The Rifleman" Flemmi, because they were informants.

But O'Sullivan said he had already made up his mind not to indict Bulger and Flemmi in the 1979 horse race fixing case, in which 21 other members and associates of Whitey Bulger's Winter Hill Gang were charged.