In 2001, actor Ashton Kutcher turned up at a woman's Hollywood house for a date.
The lights were on but no one came to the door despite repeated knocks.
Curious, he peered through a window and saw "what I thought was red wine spilled on the carpet".
"I didn't really think anything of it," Kutcher said, adding that he assumed fashion design student Ashley Ellerin had gone out with a friend, reported Reuters.
He thought she had cancelled their date because she was upset that he had not show up on time.
Kutcher was recalling what happened on Feb 22, 2001 in a Los Angeles court on Wednesday (May 29).
Ellerin, 22, was found dead at her home the next morning by a roommate.
She had been stabbed 47 times.
Kutcher was testifying as a witness at the trial of Michael Gargiulo, 43, who is dubbed the Hollywood Ripper and charged with Ellerin's murder, and that of two others.
According to entertainment portal TMZ, Kutcher called Ellerin at 8.24pm and they spoke about going out for dinner.
He contacted her at about 10pm, leaving a message that they would go out for drinks instead.
Kutcher, who met Ellerin through a friend, went to her place, arriving between 10.30pm and 10.45pm.
The actor, now 41, said when he found out the next day that she was dead, he contacted the police.
He was afraid that he would be considered a suspect since his fingerprints would be on the front door of her home.
"I went to the detectives and said: 'My fingerprints are on the door.' I was freaking out."
Gargiulo, who was arrested in 2008, has pleaded not guilty.
He is also accused of killing an 18-year-old neighbour in 1993 when he lived in Illinois.
He is also said to be behind a 2005 murder of another neighbour.
Gargiulo also reportedly attacked another neighbour in 2008 but she managed to call the police for help. Blood found at the scene led the police to link him to other attacks.
Prosecutors described what he had allegedly done as "his hobby".
Kutcher, who was calm giving his testimony for about 35 minutes on Wednesday, made his name in 2001 as one of the stars of television comedy That '70s Show.
Ten years later, he was cast in hit comedy Two And A Half Men.
In 2005, he married actress Demi Moore.
After their 2013 divorce, he wed That '70s Show co-star Mila Kunis, with whom he has two children.