Ashton Kutcher to testify against man accused of killing his ex

Ashton Kutcher is preparing to take the witness stand in the case of his then-girlfriend's grisly death, and his testimony could help convict her alleged killer.

Michael Thomas Gargiulo is currently in custody in Downtown Los Angeles, standing accused of murdering three women, including a young lady that Kutcher was seeing at the time, 22-year-old Ashley Ellerin.

On February 21, 2001, police say that Ellerin was stabbed 47 times while in the shower.

On that day, Kutcher tells police that he received a call from Ellerin asking if she should go with him to a GRAMMY viewing party. The That '70s Show actor, then 23, went by her Hollywood home later that day at 10:45 p.m.

"He was picking her up and he got to the house and knocked on the door and it was dark," Susan Zirinsky, senior executive producer of 48 Hours, told ET. "So he decides to peer in this very large window in the front of the house."

Kutcher said he saw what looked like red wine pooled on the floor before he left. Sadly, the red wine was blood from Ellerin's body, which was lying lifeless just out of his view.

Gargiulo reportedly lived just 400 ft. away from Ellerin. He is also accused of killing high school classmate Tricia Pacaccio in 1993. Pacaccio died in the Chicago suburb of Glenview.

"It wasn't until 48 Hours aired and two witnesses came forward that Chicago was able to nail him and charge him with this case," Zirinsky said.

A third alleged victim, Maria Bruno, reportedly lived less than 100 ft. from Gargiulo in Monterey Park, Ca. when she was killed on Dec. 1, 2005.

Gargiulo's pre-trial hearing is on March 20.