According to reports, the victims were largely from the Dulaim tribe, the largest Sunni Arab tribe in Iraq.
The victims were reported to have been tied up before being shot in the head.
Security spokesman Maj Gen Qassim al-Moussawi said, "A group wearing National Guard uniforms and carrying night vision equipment stormed the homes of the victims and took them to their front gardens."
"Then they handcuffed them with plastic tape and shot them in the head with guns fitted with silencers," he said.
An official at Iraq's Interior Ministry confirmed the attack and said the victims were 20 men and five women and that the attackers were in military uniform.