Near the end of more than 14 hours of questioning, Nathaniel Damian Marr acknowledged that he fatally shot a young man who he believed had killed his cousin, a Prince George's County detective testified yesterday on the opening day of Marr's murder trial.

The circumstances of the interrogation are an issue in the trial. Police arrested Marr while his attorney was trying to arrange a surrender, and they acknowledge that they ignored the offer because they wanted an opportunity to question Marr alone.

After his arrest on a warrant in a separate, nonfatal shooting, Marr waived his right to an attorney, according to Detective Sgt. Joseph McCann. During the long night of questioning that followed, Marr, 22, admitted to shooting Arthur Carroll on Dec. 2, but he offered conflicting reasons, McCann said.

At first, according to McCann, Marr said the shooting was in retaliation for the slaying four days earlier of his cousin, Ronald Anthony "Cheese" Muse, but in a written statement, Marr said he shot in self-defense.

In his opening statement and in questioning McCann and another detective, Assistant Public Defender Gary Ward did not challenge the substance of his client's statements to police, but did question whether they were properly obtained. Ward said Steven Kupferberg, an attorney for Marr at the time, had told police orally and in writing that he represented Marr and wanted to arrange his surrender.

That offer was ignored, Ward said.

"Sometimes the police exceed their authority," Ward said. "Sometimes they don't abide by the rules that govern them. This is that kind of case."

Under cross-examination by Ward, both McCann and Detective Troy Harding testified that they were aware that Kupferberg had contacted police about arranging a surrender. Once they had the suspect in custody, they both said, they notified Marr of his right to an attorney, and he never asked for one.

Harding testified that he notified Marr of his rights when speaking to him. McCann testified that he gave Marr a form describing those rights and that Marr initialed boxes indicating he was waiving them. That form was entered into evidence.

Nor did Marr object to the length of the questioning, which began about 1 p.m. Dec. 30 and ended about 3:45 a.m. Dec. 31, the detectives testified.

Marr is charged with murdering Carroll, attempting to murder a cabdriver who was injured in the attack and other related crimes. A Prince George's Circuit Court jury last week convicted an alleged accomplice, Curtis Windell Alston, 22, of first-degree murder, first-degree assault in the wounding of the cabdriver and use of a handgun in the commission of a crime of violence.

McCann testified yesterday that Marr gave the following account:

Bent on revenge, he and Alston went looking for Carroll, 22, at the Seat Pleasant home he shared with his grandmother. As they arrived, they saw Carroll leave the house and get into a taxicab. Marr allegedly told the detectives, "I knew I had to do something." Along with Alston, he fired into the cab at Carroll. But in a written statement, he said he fired only after Carroll seemed to reach for a weapon.

Alston faces up to life without parole in the slaying when Circuit Court Judge Richard H. Sothoron Jr. sentences him Aug. 12.

The Carroll slaying was one of at least 21 slayings that have occurred in and around Seat Pleasant during the last three years--violence spurred by friction among loosely knit gangs.