US actor Ashton Kutcher testified in an LA courtroom that he called on a young woman's home for a date in 2001, not realising she lay dead inside.
When the woman, Ashley Ellerin, did not answer the door, Mr Kutcher said he looked in her window and saw what he thought were wine stains on the floor.
Mr Kutcher said in court on Wednesday he was "freaking out" when he learned that Ellerin, 22, had been murdered.
Prosecutors allege Ellerin was slain by "Hollywood Ripper", Michael Gargiulo.
Mr Gargiulo, 43, is charged in the killings of Ellerin and two others.
On Wednesday, Mr Kutcher told the court he had spoken with Ellerin, a fashion design student, on the phone earlier in the day of her death to arrange a date, US media report.
The actor, best known for his roles in the sitcoms That '70s Show and Two and a Half Men, said he arrived late at Ellerin's Hollywood home to pick her up.
"I knocked on the door and there was no answer," Mr Kutcher said in court, according to AP News.
"I knocked again, and once again, no answer. At this point I pretty well assumed she had left for the night, and that I was late, and she was upset."
Ellerin was found dead in her home the next day by a roommate.
Prosecutors described Mr Gargiulo as a "serial sexual-thrill killer" and say he entered Ellerin's house before stabbing her 47 times until she was nearly decapitated.
Mr Kutcher said he tried to call Ellerin several times later in the night and could not reach her, but assumed it was bad service.
"Eighteen years ago, cell service wasn't what it was today," he said, AP News reported.
Mr Gargiulo pleaded not guilty to two charges of murder and one of attempted murder for attacks between 2001 and 2008. He is charged separately for the murder of an 18-year-old woman in Illinois.