Four Camp Pendleton-based Marines, including one known as "Psycho," have pleaded not guilty in the execution-style slayings of a fellow Marine and his wife.

Sgt. Jan Pietrzak, 24, and his wife, Quiana Jenkins-Pietrzak, 26, were found gagged, tied and shot in the head in the living room of their Winchester home on Oct. 15. Investigators said the house had been ransacked and a fire had been set, an apparent effort to destroy evidence.

The not guilty pleas were entered Thursday at Superior Court by Lance Cpl. Emrys John, 18, of Maryland; Lance Cpl. Tyrone Miller, 20, of North Carolina; Pvt. Kevin Cox, 20, of Tennessee, and Lance Cpl. Kesaun Sykes, 21, of California.

John and Miller worked for Pietrzak, who was a helicopter airframe mechanic.

Miller, Cox and Sykes, known as "Psycho," told police they went to the home to rob Pietrzak. All four said his wife was sexually assaulted, according to an investigator's affidavit.

The Marines are charged with two counts of first-degree murder and special-circumstance allegations of committing multiple murders, committing the crime during a robbery and rape by instrument.

John, whom prosecutors believe shot the couple, is also charged with a special-circumstance allegation of using a firearm to inflict great bodily injury or death.

Prosecutors have not decided whether to seek the death penalty.