Seven Navy SEALS -- who have been granted immunity -- and up to 13 additional witnesses are expected to be called to testify against a decorated chief special warfare operator accused of multiple war crimes stemming from a 2017 incident in Iraq, prosecutors said.

Edward R. Gallagher has been charged with killing a wounded ISIS prisoner of war, who Navy prosecutors believe was about 15 years old, and firing sniper rounds at innocent civilians during a tour in Iraq. He has pleaded not guilty to the charges, which include premeditated murder.

During a motion hearing Friday, prosecutors said witnesses told NCIS investigators Gallagher bragged about killing up to 200 people – “three a day” he allegedly said – during his 2017 deployment to Mosul, Iraq, the San Diego Tribune reported.

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Prosecutors said they also have a knife recovered from Naval Special Warfare Group 1 in Coronado that's tested positive for DNA. They did not reveal whose DNA was allegedly found on the knife or state definitively that it was the murder weapon used to stab the ISIS fighter.

Gallagher, 39, fought in Iraq and Afghanistan several times during a nearly two-decade military career, but it was during his last combat deployment, in 2017, that prosecutors say he lost his way and “decided to act like the monster the terrorists accuse us of being.”

Prosecutors allege Gallagher stabbed an ISIS fighter who was receiving medical treatment from SEAL medics and then posed for photos with the corpse, operating a drone over the body, and, sometime later, celebrated his reenlistment next to the dead militant. Gallagher allegedly sent one of the images to a fellow SEAL with the caption: “I got him with my hunting knife.”

At Friday’s hearing, Gallagher’s defense attorneys asked the judge, Navy Capt. Aaron Rugh, to suppress some aspects of the witnesses’ expanded testimony, particularly the number of people Gallagher allegedly bragged about killing, the Tribune reported. The judge denied the motion.

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His lawyers also asked that evidence seized from his three cell phones be suppressed. They said their witnesses will testify the fighter was either mortally wounded when he was brought to Gallagher or was alive when Gallagher left the scene.

A Navy officer who supervised Gallagher was charged last week with various offenses tied to the case, including allegations he conducted the SEAL’s alleged battlefield reenlistment ceremony and encouraged enlisted personnel to pose for photos with the corpse.

Lt. Jacob Portier also is accused of failing to report a war crime, destroying evidence and impeding the investigation against Gallagher.

Portier's attorney, Jay Sullivan, said Portier will plead not guilty to all charges at a later date, which is allowed under military court rules. He said his client, who was the officer in charge of Gallagher's platoon during the deployment, is innocent. He also said the re-enlistment ceremony was done legally in a war zone where there may have been casualties nearby.

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"I can tell you he certainly never ordered anybody to appear in any photos with a dead ISIS fighter," Sullivan said after the arraignment. "I can tell you that a re-enlistment ceremony was done on the battlefield and for a Navy SEAL nothing could be more proud and honorable than re-enlisting to serve your country on the battlefield."

Gallagher, who has been jailed since his arrest on Sept. 11, is set to stand trial Feb. 19.

Fox News' Greg Norman and the Associated Press contributed to this report.