Dela Cruz had been charged with unpremeditated murder and could have received up to life in prison for the deaths of five Iraqi civilians in the November 19, 2005, killings.
He has been granted immunity from prosecution and must testify at upcoming hearings for other marines charged in the Haditha case.
Dan Marino, Dela Cruz's lawyer, declined to comment.
On the day of the killings, a marine squad was in Haditha, a town in Anbar province, when their convoy was hit by a roadside bomb killing one marine. In response, the marines raided several homes and killed 24 Iraqis, including women and children.
Dela Cruz and three other marines were charged in December with unpremeditated murder in the deaths.
"The chances of success [in Iraq] are essentially zero because the Iraqi people have no voice"
Non Sequitur, Cadiz, Spain
Send us your viewsThe marines say they believed they were under attack in the wake of the roadside bomb blast and followed procedures to defend themselves.
Other marines granted immunity include an officer who told troops to raid a house and a sergeant who took photographs of the dead but later deleted them from his camera, according to the Associated Press.
The immunity orders ensure that any testimony the marines volunteer cannot be used against them.
Lieutenant-Colonel Sean Gibson declined on Friday to comment on individual cases due to the ongoing nature of the investigation.
In a separate investigation, a US army general concluded the Marine Corps chain of command in Iraq ignored signs of "serious misconduct" in the Haditha killings, The Washington Post reported.
A report by Major-General Eldon Bargewell found officers may have willfully ignored reports of the civilian deaths to protect themselves and their units from blame.
Bargewell concluded that commanders fostered a tendency that devalued Iraqis to the extent that US soldiers considered the deaths of innocents insignificant.
The report, now unclassified, focuses on the reporting of the Haditha incident and the training and command climate within the Marine Corps leadership.
It does not address the November 19, 2005, incident in detail.