The prosecution says Shawn Gaston fired shots that killed Valadez in 2009, and they say after the shooting, Gaston went and partied with friends.
Cook County State's Attorney Anita Alvarez is the lead prosecutor for the case against Gaston, her first since being elected in 2008.
Valadez comes from a family of police officers, who were all in the courtroom and visibly distraught during the testimony of his partner, Tom Vargas.
Wearing the exact same plain clothes, bulletproof vest, duty belt and badge the night Valadez was killed, Vargas choked up as he told the jury how he saw Valadez fall when two rounds of bullets aimed their way came from a passing car.
Prosecutors say Gaston fired the shots.
As he rushed towards his partner, who was lying face-up with a bullet wound to his head, a tearful Vargas testified Valadez's eyes were open but he hever responded.
The two officers were responding to a call of shots fired in the Englewood neighborhood.
Alvarez said police found Gaston's car parked near his home. After a search, three different weapons were found in the truck. She said all matched shell casings from the scene.
Using physics, defense attorneys will argue the bullets could not have come from Gaston's car.
"The bullets came from the street when the car was coming down the street and he was facing in the opposite direction," defense attorney Michelle Gonzalez said. "(The shots) would have hit the right side of his head if they were coming from the car."
During his opening statement, defense attorney John Carroll referred to the prosecution of his client as a lynching. Gaston is the first of three to be tried in the case.