Nothing like it has ever been seen in D.C. Superior Court: nine young men and one woman, most of them neighbors, on trial for a murder that authorities say is almost unspeakable and unexplainable in its brutality.

Courtroom No. 1 has been transformed into something resembling a classroom, with the accused and their lawyers sitting in neat rows. For the past week, the courtroom has been the scene of wrangling among defense lawyers and prosecutors over matters of evidence and law. This week, a jury will begin to hear evidence on how and why Catherine L. Fuller was killed.

Prosecutors charge that Fuller, 49, a slight mother of six, was killed because she fought back when a group of young people, who had staked out a Northeast Washington territory as their own, tried to steal her coin purse. So far, two other defendants have pleaded guilty to charges in Fuller's death.

In the last week prosecutors have outlined their case against the 10. Fuller, they say, was beaten and kicked, stripped nearly naked as the coin purse was ripped from her bra, and dragged over glass into an abandoned garage near her home. There, a foot-long pole was thrust into her rectum as a group of about 30 young people watched or participated, prosecutors say. Poster-sized photographs of her battered body were held aloft in the courtroom last week as a judge determined which could be admitted as evidence during the trial.

Defense lawyers and prosecutors predict four to six weeks of testimony. Some of the 10 are expected to argue they were not present in the alley behind the 800 block of Ninth Street NE, while others may argue they were there but did not participate in the beating.

"None of us will dispute that what happened to Mrs. Fuller was horrible," said one defense lawyer, who represents a young man who maintains he was not there. "The issue will be are the right people on trial."

But no matter the trial's outcome, the central questions will most likely go unanswered: How could such a crime occur over a coin purse, and why did no one stop it? Prosecutors and police officers remain baffled, even after interviewing more than 400 people.

"I guess the answer to that question occurs somewhere between the intersection of psychology and sociology," said one high-ranking prosecutor who has watched the case develop during the last year. "You could say that it had something to do with getting carried away with the crowd . . . but I don't find that a satisfying answer. When they rammed the pipe up her rectum, that was just gratuitous infliction of pain."

Police and prosecutors say they do not believe the group set out to kill Fuller Oct. 1, 1984, and they acknowledge that some of the accused were less involved than others. They believe that many of the accused, ranging in age from 17 to 26, were members of a tightly knit gang of dropouts and unemployed youths -- the 8th 'n' H Crew -- who decided that afternoon to rob someone. Once the beating started, the group lost control, police have theorized.

Last week when one youth pleaded guilty to murder charges, Assistant U.S. Attorney Jerry S. Goren sketched out a scenario showing some defendants more involved than others in the brutal aspects of Fuller's death. Prosecutors argue that District law allows a first-degree murder conviction if a person participates in any way in a felony that leads to a murder, even if the death is accidental. All 10 have been charged with armed robbery and kidnaping in addition to first-degree murder.

The case presents dilemmas for everyone, including Judge Robert M. Scott, who is presiding. Although all ten are being tried together, there are in essence 10 separate trials. This means that the 63-year-old jurist, who has a reputation for bluntness and handing out tough sentences, must not only ensure the fairness of the overall proceedings but also make sure each individual defendant is treated fairly.

For prosecutors Goren -- who for nearly a year has spent almost all his working hours investigating the case -- and Jeffrey Behm, the task is to convince 12 people that such a large group could participate in the murder of one woman, weighing only 100 pounds.

Goren, whose easygoing manner is popular among many of the defense lawyers, has played an unusual role in the case, working side by side with detectives.

"Gerry is the computer in this case," said Detective Ruben Sanchez-Serrano, who also has worked on the case nearly every working day for a year.

For the defense lawyers, the case presents other problems. Many say they fear that the brutal nature of Fuller's death may so offend and anger the jurors that there will be little room for ambiguity. Some defense lawyers say they also are concerned that the defendants will be judged en masse, that if one is convicted of first-degree murder all will be convicted. Nearly every day for the last week and a half, defense lawyers have asked unsuccessfully for separate trials. One defendant, whose attorney withdrew, will be tried later in separate proceedings.