Tatum, 26, begins a preliminary hearing Monday to determine if he should be tried for murder. The Edmond, Okla., native is the second of three enlisted Marines in the case to face a hearing to assess whether his charges should be referred to a court-martial.
"Knowing what I know now, I feel badly about killing Iraqi civilians who may have been innocent," Tatum told Navy investigators in March 2006. "But I stand fast in my decisions that day, as I reacted to the threats that I perceived at that time."
Tatum is charged with the unpremeditated murder of two girls in one house and the negligent homicide of two men, a woman and a child in another home. He is also charged with assaulting two men.
If convicted of murder, he faces life in prison.
The killings took place Nov. 19, 2005 after a roadside bomb killed a Marine. In the aftermath of the blast, a Marine squad went house to house looking for insurgents, but instead killed Iraqi civilians, including women and children in bed.
Tatum's attorney, Jack Zimmerman, said his client acted appropriately and raided the houses after taking enemy fire from the vicinity.
"Lance Corporal Tatum responded precisely like he was trained to respond to a hostile situation," Zimmerman said Friday. "He certainly didn't intend to commit any crime."
According to investigative documents, Tatum went with several other Marines to a house, where he said he and Cpl. Hector Salinas threw grenades into a room after hearing what they thought was the metal-on-metal sound of an AK-47 being readied to fire. The squad leader, Staff Sgt. Frank Wuterich and another Marine, Lance Cpl. Humberto Manuel Mendoza, then fired into the room.
Tatum said he joined in the firing and shot at least four people at a distance of about 20 feet. He said he did not positively identify those he shot as insurgents because he considered the entire house to be hostile.
Mendoza has been given immunity from prosecution and may testify at Tatum's hearing.
The preliminary hearing for Wuterich, who is charged with murdering 18 Iraqis, is set for Aug. 22.
In another house, Tatum said he may have shot as many as five people. He said he determined the house was hostile because Wuterich began firing his weapon.
Besides the three enlisted Marines charged with murder, four officers are charged with dereliction of duty for failing to investigate the deaths. A hearing officer for Lt. Col. Jeffrey Chessani, the highest-ranking Marine charged, has recommended a court-martial.
Last week, the investigating officer for squad member Lance Cpl. Justin L. Sharratt, who is charged with murder, said the government's evidence was insufficient for a court-martial and recommended dropping charges against Sharratt.
Recommendations are nonbinding; the final decisions about whether charges should go to trial rests with Lt. Gen. James Mattis, the general overseeing the case.