In a crowded room, nobody notices a man being knifed to death. Doors that are supposed to be locked are not. And it's hard to say who is less attractive - the accused killer, the victim or the witnesses.

But that is the reality of prison murder cases.

Those challenges were on display last month when an Anne Arundel County jury stunned prosecutors by acquitting a convicted murderer of the stabbing death of a fellow inmate.

"We put on everything that we had," said Assistant State's Attorney Frederick M. Paone, who was seeking the death penalty against John Ashby, an inmate at the Maryland House of Correction Annex in Jessup. "If this would have involved a civilian, it would have been a strong case."

When prosecutors try someone for murder behind bars, they face troublesome witnesses, crime scenes tainted by the need to control prisoners, thin investigations, and victims whose criminal records can be laid bare to the jury.

Such cases can be enormously expensive and time-consuming - one Anne Arundel County case ate up three years of staff time, prosecutors say. And a conviction might seem little more than a moral victory in a case where the defendant already is serving a life sentence.

But prosecutors say that bringing charges helps to maintain order, making it clear to prisoners that authorities are unwilling to cede control to inmates who would rule through intimidation. On a more basic level, the state is seeking justice for someone who has been killed.

"Don't people still have rights even though they are still in prison?" asked M. Kenneth Long, state's attorney for Washington County, home of Maryland's largest prisoner population, about 8,000.

Prison murder remains relatively rare - 19 inmates were killed during the past 5 1/2 years in Maryland. About half of them were in the seven Jessup facilities on the western edge of Anne Arundel County, home to 6,000 of the state's most violent offenders.

Not all prison killings lead to court. Since 1997, charges have been brought in four of nine Jessup-area homicides behind bars, including one case from Patuxent Institution in Howard County.

The results have been mixed.

Ashby's case, the most recent, resulted in a not-guilty verdict on all counts. Another case ended in a mistrial, which was followed by a guilty verdict. In the Howard County case, the defendant was found mentally incompetent for trial. Yet, three prisoners in another homicide were indicted last month.

Prosecutors can face daunting obstacles in bringing these cases to court. A case in point is the 1999 trial of David Dodson, accused of killing a convicted thief in what was supposed to be a locked infirmary at the Maryland House of Correction in Jessup.

Anne Arundel County Assistant State's Attorney Eileen A. Reilly lacked physical evidence, including a murder weapon, which authorities say might have been thrown out.

Reilly was counting on three witnesses, but there were problems. Two of the witnesses - a convicted killer and a serial sex offender - lacked credibility with the jury, something that's typical in such cases, prosecutors say.

Her third inmate witness was beaten up, moved to another prison and promised immunity, but refused to testify, saying he preferred to live out the few remaining years of his prison term. He was threatened with contempt, for which the punishment was jail.

"But, you know, he was already incarcerated," Reilly said. "You don't want to be a snitch in prison."

Jurors found Dodson not guilty, although the outcome made little material difference - he was already serving a triple life sentence for multiple murders.

Defense attorneys reject the idea that they have an unfair edge in these cases. They say that prosecutors can offer prisoners incentives to testify, and note that their inmate clients and defense witnesses are not readily endearing to jurors.

But defense attorneys agree that they have some cards to play. The defense can discuss a victim's past, while the rules of evidence severely limit the amount of background about the defendant that can be presented by the prosecution.

In the Ashby case, the defense portrayed the victim as a one-man crime wave behind bars - while acting district public defender Keith J. Gross coached his client to make a better impression in court by not shaving his head and by hiding his large physique in a suit from Goodwill.

And some cases are simply weak, defense attorneys say.

Mary Jo Livingston, the assistant public defender who represented Dodson, lists what she says were a number of flaws in the prosecution's case.

In that case, doors within the prison were unlocked. Blood in the victim's cell matched neither Dodson nor the victim. There were questions about what guards on the tier were doing at the time of the slaying, when dozens of people might have had access to the victim, as well as conflicting testimony.

"They did not prove their case because of the holes," said Livingston.

Such complexities add to the cost of prosecuting prison murder cases.

The burden is so great that after one 1999 case, Anne Arundel County State's Attorney Frank R. Weathersbee refused to seek new indictments in prisoner-on-prisoner crimes until state officials gave him about $60,000 toward prosecutions. In 2000, he got the first and only such one-year grant.

Prosecutors around the state mounted an effort in the General Assembly last year to win similar payments. They failed but vowed to try again.

Public defenders counter that their local offices don't automatically get a budget boost for defending prisoners. And some suggest that prosecutors take a more critical look at whether it's worth the time and expense of bringing a case.

Prosecutors reject suggestions that there's little to gain from prosecuting such cases.

Daniel Andrews is one of two Anne Arundel prosecutors who tried Andre Logan in the fatal stabbing of Johnny Smith in the Annex's activity area in 1997. The killing touched off a near-riot, complicating the collection of evidence. Witnesses were reluctant to testify. Logan's first trial ended in a mistrial.

Eventually, Logan was convicted and received a life sentence.

"What do you get out of it? You get a conviction for the person who died - even if we do not end up adding a day to [the defendant's] sentence," Andrews said.