When prosecutors built their murder case against Keith Little — accused of stabbing his boss more than 70 times in a hospital boiler room — they were counting on a key piece of evidence: a glove at the scene that they said bore traces of Little’s DNA on the inside and the victim’s blood on the outside.

On Tuesday, moments before Little’s trial in the death of Roosevelt Brockington was to begin, that part of their case took a major blow.

Montgomery County Circuit Judge Marielsa Bernard ruled that jurors couldn’t be told about the blood after a variety of tests gave different results. Prosecutors indicated that they would appeal the ruling, which experts said might make their case more difficult to prove.

“It is a very significant ruling,” said James Papirmeister, a longtime local defense lawyer and former prosecutor who is not involved in the case but has followed it. “That was a compelling piece of evidence. Jurors are primed, from various television shows such like ‘CSI,’ to expect forensic evidence.”

Prosecutors declined to comment further after court proceedings ended. The matter is expected to be taken up Wednesday morning.

The ruling followed another that went against prosecutors in the case, which has drawn attention because of the horrific violence and the fact that Little had been accused of killing another co-worker in 2003. A D.C. jury found him not guilty.

Prosecutors had wanted to tell jurors about that case, arguing that Little had a “common modus operandi” and that “the evidence of both murders suggests that the defendant solves problems at work by brutally killing the person he perceives as a threat.”

But Little’s attorneys argued that doing so would prejudice the jury, that there was no clear evidence that Little was involved in the 2003 killing and that the two killings were not done in a similar way.

“In 2003, according to the state [prosecutors] chart, the victim was shot six times. In 2011, the victim received seventy stab wounds,” wrote defense attorneys Ronald Gottlieb and Adam Harris. “Six gunshot wounds is not out of the ordinary. Seventy stab wounds, on the other hand, is uniquely excessive.”

Bernard ruled in October that prosecutors would not be allowed to discuss the 2003 case during the trial.

Opening statements in the trial got underway just after noon Tuesday. Prosecutor George Simms said Little hated Brockington, his boss in the basement boiler room at Suburban Hospital in Bethesda.

Little worked a second job at the federal courthouse in Greenbelt, according to Simms. That led to friction between the men when Brockington set Little’s hospital shift schedule in a way that made it impossible to keep the other job, Simms said.

At some point, Simms said, people heard Little making threats about Brockington. “I’m going to get that guy,” Little said repeatedly, according to Simms.

Then came a performance review in November 2010.

“Your tardiness will not be tolerated in the coming year,” Brockington told Little, according to Simms. “And if it is not turned around immediately, there will be disciplinary action taken up to and including termination.”

Brockington was killed the morning of Jan. 1. The evening of Jan. 5, a hospital co-worker saw Little washing something with chemically treated water, walked over and saw a pair of gloves and black ski mask that police say were used by the killer.

Little told the worker not to worry and buried the items in a trash can, according to prosecutors, but police later found them. One of the gloves showed signs of DNA from Little and Brockington, Simms said. But defense attorneys could say the two men simply had touched the same glove.

Simms called the washing and disposal of the gloves and mask “the actions of a guilty person.” But Gottlieb, the defense attorney, said in his opening statement that jurors would not find out who killed Brockington or why Little was charged.

“This case is about two mysteries,” Gottlieb said.

During the trial, jurors will have to wrestle with an odd detail. They were shown a photo of the weapon used to kill Brockington, a 12-inch knife, and they could see a red swastika on the handle. Little is African American, perhaps raising questions about why he would have a knife bearing a Nazi symbol.

When first questioned by detectives, Little spoke of a former manager — a Caucasian he said came across as racist and didn’t get along with Brockington, another African American.

“Maybe I shouldn’t be saying this, but, you know, I think it was a little tension with one of the guys that used to, used to be the supervisor here,” he said. “I don’t really think the guy liked black people.”

That interview, a transcript of which was made public in a pretrial hearing, has not been presented to jurors yet.

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