Cruz was testifying in a preliminary hearing on Wednesday into charges against Captain Randy Stone, who is accused of failing in his duty to investigate and report the 24 deaths.
Cruz said he also pumped bullets into the five dead men before Wuterich shot them again in the torso and head, and told him to blame the Iraqi army.
"I just held my weapon up and shot them. I know they're dead, but I wanted to make sure they were dead," he told the hearing.
Cruz said he agreed to go along with Wuterich's story because he was "trying to cover up [for] the squad".
But his testimony had no direct link to Stone.
The preliminary hearing will determine whether Stone's case advances to a full court-martial later this year.
Prosecutors allege marines patrolling Haditha were angered by a comrade's death and had gone on a shooting rampage against unarmed men, women and children.
Three of the marines are facing murder charges while four others have been charged with a cover-up.
All seven accused said they were following orders.