Garrido was set to be prosecuted in the case but the charges were dropped when the girl refused to testify, Orman said.
The girl and a friend met Garrido, then 21, and another man near the public library and joined them in a car, where they were given drugs, he said.
Details on the case were spotty because so much time had passed. However, authorities recently reinterviewed the girl and were able to piece together that she awoke at the motel and was raped repeatedly before her parents found her, Orman said.
It was the third sexual assault linked to Garrido by authorities.
Garrido, 58, and his wife, 54-year-old Nancy Garrido, pleaded not guilty to 29 counts of kidnapping, rape and false imprisonment in the disappearance of Jaycee Dugard 18 years ago. Both suspects are being held without bail.
Susan Gellman, Phillip Garrido's public defender in El Dorado County, did not immediately return a call seeking comment on the 1972 case.
Orman said the chances of Garrido being charged again in the 1972 case were "very slim."
Police say the Garridos held Dugard captive in a backyard encampment of tents and sheds in Antioch, and Garrido fathered two children with Dugard.
He was previously convicted in the 1976 kidnapping of a casino worker in Reno, Nev., who said Garrido raped her in a storage locker before police found them. He spent about 11 years in federal prison and was paroled in 1988. Dugard was abducted in 1991 near her South Lake Tahoe home.
Earlier Thursday, Jaycee Dugard's aunt read a brief statement to reporters saying her niece remembers her family and is enjoying getting to know her younger sister, who was a baby when Dugard was kidnapped.
Tina Dugard spoke to reporters at the FBI's Los Angeles office, describing her niece's reunion with her mother and sister.
"The smile on my sister's face was as wide as the sea. Her oldest daughter is finally home," Tina Dugard said.
Tina Dugard said her niece's daughters appeared to be bright and educated, even though they did not attend school.
"Jaycee did a truly amazing job with the limited resources and education that she herself had, and we are so proud of her," Dugard said.
The family's location has been a closely guarded secret since the 29-year-old woman reappeared last week. She was 11 when she was allegedly kidnapped.
Tina Dugard said the family has been spending time in a secluded place, reconnecting and getting to know each other again. "Not only have we laughed and cried together, but we've spent time sitting quietly, taking pleasure in each other's company," Dugard said.
Tina Dugard took no questions from reporters and did not comment on the investigation into her niece's abduction. A spokeswoman for the Dugard family, Erika Price Schulte, said they would have no further public comment for now.
The modest Riverside home of Jaycee Dugard's mother, Terry Probyn, was quiet on Thursday.
Neighbors strung pink ribbons around tree trunks up and down the street to show their support. They also spruced up Probyn's front lawn and lent a hand with gardening while she has been gone.
"When you close your doors, you have absolutely no idea the lives and struggles behind those doors," said neighbor Lisa Brown, who does not know Probyn but wanted to show her support.
Several neighbors said they didn't know about Probyn's past until media crews started showing up on the street. They said she moved into the house several months ago.
Rindels reported from Los Angeles. Associated Press Writer Amy Taxin in Riverside contributed to this story.