Prosecutors argue it was an unprovoked, cold-blooded murder, but the defendant’s attorney said his client was acting in self-defense.

This week it will be up to jurors to decide what happened to Lawrence resident Robert Martin during the early morning hours of April 4, 2004, at a small party inside a third-floor Jefferson Commons apartment.

“Lafayette Cosby shot Robert Martin three times, without provocation or justification,” Assistant Dist. Atty. Dave Zabel told jurors during opening statements Monday morning.

Cosby is standing trial, charged with premeditated, first-degree murder in connection with Martin’s death.

“Twenty-eight-year-old Robert Martin spent the last moments of his life on a couch, defenseless and unarmed,” Zabel said.

But he wasn’t alone.

Prosecutors said they would present evidence over the course of the trial that seven people actually witnessed Cosby gun down Martin inside the small apartment, including Alrick Johnson, whose testimony took up much of the day Monday.

Johnson, who goes by the street name “10-10,” took the stand, describing how he was seated next to Martin on a couch when the fatal shots were fired.

“I saw a glimpse of something coming toward us,” Johnson said, “then I saw fire and heard boom, boom, boom. My ears were ringing because it was so close. I thought I was shot.”

Johnson testified that many of the party-goers fled the apartment after the shooting. Police were not notified until several hours later, and by then Martin had bled to death from the three gunshot wounds, according to Douglas County Coroner Erik Mitchell, who testified late Monday afternoon.

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Mitchell described how Martin was shot once in the left side of his chest, damaging his left lung and heart, once in the middle of his chest, piercing his liver and spinal column, and once in the right side of his chest — that bullet traveled through his right lung.

Mitchell also discovered what he called “patterns of strippling” on Martin, proving that the victim was shot at close range.

“Strippling is marks caused by materials other than the bullet that come out of a firearm,” Mitchell said. “Pieces of powder and other materials come out at approximately the same speed.”

Martin had such marks on his neck, face and the backs of his forearms. Mitchell testified that proved the shooter was less than 3 feet away from the victim when the shots were fired.

Defense attorney Greg Robinson said during his opening statements that he would not argue whether Cosby pulled the trigger. In fact, Robinson said his client would take the stand in his own defense.

“Mr. Cosby is going to tell you what occurred,” Robinson said. “There’s no question that Mr. Cosby shot Robert Tyrone Martin. The question is why?”

Robinson plans to show the jury that his client acted in self-defense, claiming Martin was an “uninvited guest” at the small gathering.

When Martin arrived at the party that night, witnesses said he went into a back room with Cosby. Robinson is prepared to argue that only his client knows what unfolded between the two before the shooting.

“Cosby acted in full self-defense of his friend, Alrick Johnson,” Robinson said.

Robinson told jurors during his opening statements that his client was afraid for Johnson’s safety, believing that Martin was upset and blamed Johnson for sending him to jail on an unrelated case.

“Lafayette Cosby says he saw the barrel of a gun and if he did not act, his good friend was going to be shot and killed,” Robinson said.

Testimony is scheduled to continue this morning.

Cosby already has been once acquitted of murder charges. A jury found Cosby acted in self-defense in a 1997 stabbing at Colony Woods Apartments, 1301 W. 24th St.

The victim in that case, David E. Walker II, happened to be one of Robert Martin’s friends.