Ashton Kutcher thought he saw red wine on his girlfriend's floor. It was actually her blood.

Ashton Kutcher is preparing to testify in the trial of an alleged serial killer, who is accused of murdering his then-girlfriend in 2001.

The trial of Michael Gargiulo is set to begin in Los Angeles this week, where Kutcher will be called to the witness stand to recall the night of February 21, 2001, when his girlfriend Ashley Ellerin was brutally stabbed to death in her Hollywood Hills home.

At the time, Kutcher, 23, was known for That 70s Show and Dude, Where’s My Car? and was dating 22-year-old Ellerin, a fashion student and part-time exotic dancer.

He has not spoken publicly about his relationship with Ellerin, but the actor provided a testimony to police about the night of her death.

According to the testimony, Kutcher had received a phone call from Ellerin earlier in the night, trying to set a date for them that night. Kutcher turned her down - he was attending a Grammys viewing party at a friend's place - but he said he would visit her later.

He then tried to call her two times throughout the night, but she didn't pick up. Assuming she was upset with him and wanting to make amends, Kutcher drove to her house behind the Grauman's Chinese Theatre on Sunset Strip around 10.45pm.

All the lights were on, and Ellerin's maroon BMW was parked outside. Kutcher then knocked on the door and Ellerin did not answer. He called out to her and still received no answer.

Kutcher peered through the front window, telling police he saw red wine stains on the floor.

Kutcher simply assumed Ellerin was mad at him and left the apartment. What he couldn't have known, however, was that Ellerin was dead inside the apartment. She had been brutally murdered, her throat slit with such force she was almost decapitated, her body punctured with dozens of stab wounds. She was later discovered by her roommate, Justin Peterson.

Ellerin's murder remained unsolved for seven years until 2008 when police connected her case to Michael Gargiulo. Gargiulo - also known as the Chiller Killer - is currently accused of murdering two other women and he's possibly responsible for as many as 10 murders.

Ellerin had invited Gargiulo, now 43, to a few parties at her house after he moved in next door, and her friends had earlier expressed concern about his growing fixation on her.

Peterson, Ellerin's roommate, confronted Gargiulo about parking outside their house at odd hours and Gargiulo told him he was hiding out because the FBI was after him in connection with a murder in Chicago.

Gargiulo evaded authorities for years after Ellerin’s murder, before he was arrested in 2008 for the attempted murder of Michelle Murphy, another one of his neighbours.

Police then connected Gargiulo to the murder of Tricia Pacaccio on Aug 14, 1993, in his hometown of Glenview, Illinois. It's believed this is what he was referring to when speaking to Peterson.

After moving to California, he allegedly committed two murders - of Ellerin and another woman, Maria Bruno - before attempting to kill Murphy.

The trial for all three murders, as well as Murphy’s attempted murder, begins on May 2 at the Criminal Justice Center in Los Angeles.

Ellerin’s childhood friend Carolyn Murnick described her as "a real person with a childhood and people around her who loved her and still remember her".