"He is adamant he was not involved," said Gargiulo's attorney, Anthony Salerno.
No decision has been made on whether to seek the death penalty, said Jane Robison, a spokeswoman for the district attorney's office.
Ellerin was found dead in February 2001 in her Hollywood Hills home. Bruno was fatally stabbed in her apartment in December 2005.
Gargiulo pleaded not guilty to the charges when he appeared in court Thursday as part of a separate case in which he is charged with attempted murder and burglary in an April 28 knife attack on a Santa Monica woman.
Gargiulo was arrested in July and is being held in lieu of $1.1 million bail in connection with the attack on the Santa Monica woman who awoke to an intruder wielding a knife. Through his lawyer, Gargiulo also has denied involvement in that attack.
In all three cases, Gargiulo lived near the victims, and the women were stabbed in their homes, police said.
Authorities suspect Gargiulo may have committed similar attacks.
"We're interested in hearing from anyone who's even loosely connected with this guy," said sheriff's Detective Mark Lillienfeld.
In the 2001 case, Kutcher told police he went to pick up Ellerin, a fashion student-model, for a post-Grammy Awards party, but she did not answer the door.
He checked a back window and spotted what he thought were red wine stains on the carpet and then left. Her body was discovered the next day by a friend.
Kutcher starred in the television series "That '70s Show" and is married to actress Demi Moore.
Police have said Gargiulo might be linked to another killing that occurred in 1993 in the Chicago suburb of Glenview. High school senior Tricia Pacaccio was found stabbed on her doorstep, clutching her door key.
John Gorman, spokesman for the Cook County State Attorney's Office, said Thursday the investigation remained open and he could not comment further.
Salerno said his client, a native of Illinois, denies any connection to that crime.