Jury selection in the death penalty case took about a month to complete.
Family of the five victims packed into the courtroom for day one of the trial.
At the beginning of Monday's proceeding, the judge asked 33-year-old Cooper if he had agreed to allow his attorneys to admit to the jury that he did certain things.
Cooper answered, "Yes," and then told the judge, "We're ready to go."
As prosecutor Adam Moyers described each of the murders, the family members listened intently and some cried.
One of Cooper's three attorneys, Stephen Freedman, presented the defense's opening statement. He immediately told jurors that the defense will not dispute that Cooper killed five men across Wake County from May 2006 to October 2007.
He mentioned his client's mental condition, claiming Cooper suffers from post traumatic stress disorder.
Freedman described Cooper's violent upbringing at the hands of a violent and sadistic father.
He ended by telling the jury that Cooper's defense is called "diminished capacity," which he says caused Cooper to kill people not premeditated but as a reflex."When you suffer from what he suffers with … you respond as a reflex," Freedman said. "You respond as an impulse without thinking without considering anything."
Cooper was arrested after a November 2007 bank robbery in Garner.
According to prosecutors, Cooper might still be on the loose if it wasn't for eyewitness John Pendergraft.
"Sam Cooper was trying to get in the teller door, but he told him he couldn't do that," Pendergraft said. "So he jumped the counter and threw the gun in the man's face."
Pendergraft, a truck driver, says after he saw the masked man point a gun at a Garner bank manager, he and other customers fled the building, but Pendergraft didn't go far.
"I went to my truck, unlocked the door," he said. "I called 911 and I grabbed my pistol."
Pendergraft followed the bank robber who was driving the minivan.
Dash cam video of the November 2007 pursuit was shown to the jury as part of the evidence on Monday.
When Cooper finally jumped and ran, money spilled out of a bag onto the hood of the police car. He was quickly captured and when the SBI tested his gun they linked it to deaths of five men in the previous year and a half - all but one killed in robberies.
Cooper is charged with five counts of murder and attempted murder and robbery in Wake County. He also has been charged with attempted murder in Rockingham County.