CHICAGO (CBS) — Cook County prosecutors have begun laying out their case against an alleged gang member accused of killing a Chicago police officer two years ago.

Officer Alejandro Valadez was 27 years old when he was shot to death in 2009. He had been on the force for only three years and was working undercover duty on the South Side on June 1, 2009, investigating reports of gunfire when Shawn Gaston, 22, allegedly gunned him down on the 6000 block of South Hermitage Avenue.

As CBS 2′s Mike Puccinelli reports, Gaston’s trial began Tuesday with opening statements and testimony from the prosecution’s first witness.

Chicago Police Officer Alejandro Valadez (Credit: CPD)

Prosecutors alleged that, as Valadez and his partner were questioning several residents about the reported gunfire, a vehicle drove up and Gaston opened fire, striking Officer Valadez once in the leg and once in the head, according to prosecutors.

Gaston and Kevin Walker, who was the driver, were arrested about an hour after the shooting. They were also responsible for the original gunfire Valadez was investigating, officials said. Walker and Gaston are both charged with first-degree murder.

Prosecutors said a bullet casing was found stuck on the car that Valadez was in.

Cook County State’s Attorney Anita Alvarez is the lead prosecutor in the case.

But defense attorneys said Gaston’s DNA wasn’t found on the murder weapon and that the wound to Valadez’s head appears to have come from above, making it impossible for their client to have shot Valadez from a moving car.

Outside of court, defense attorney Michelle Gonzalez said, “The trajectory is coming from above, so a car passing down that street … a person firing from that car couldn’t have hit (Valadez) from above. It would have to be straight on.”

The murder weapon and two other guns were found in the car, which belonged to Gaston’s mother.

Defense attorneys suggested that a couple of rogue officers might have planted the guns which were found in the vehicle after it was towed from the scene and then moved a second time.

The main witness on the stand Tuesday was Valadez’s partner, Thomas Vargas. He broke down when describing how Valadez was lying on his back with his eyes open, unable to respond as the life ebbed from his body.

During that testimony, much of the courtroom was in tears. But Gaston didn’t show any obvious emotion.

More than 50 uniformed Chicago police officers packed the courtroom Tuesday for opening statements and the beginning of testimony in the case.

The crowd was so large that many people had to be turned away. Others were forced to stand along the walls of the courtroom.

One juror was removed from the panel before the trial began Tuesday. She told a bailiff that she didn’t think she could remain impartial because she thought Gaston was guilty.