Karen Fraser lost a daughter and unborn grandchild when intellectually-impaired Krystal vanished, likely murdered, in Victoria a decade ago.
As police announced a $1 million reward for information about what happened to the 23-year-old, a tearful Ms Fraser on Wednesday told the guilty party to "grow a conscience".
"She was our daughter, she had a brother and sister and grandmother," she told reporters at a police press conference before directly addressing her killer.
"You took all that away from us and we don't know what happened. Grow a conscience."
Krystal was just a couple of days away from giving birth when she disappeared from Pyramid Hill, in the state's central north, on June 20, 2009.
Missing Persons Squad Detective acting Inspector Julian Horan said new information received earlier this year had sparked fresh lines of inquiry into a person previously questioned over Krystal's disappearance.
"The information is promising, however sadly leads us to look at the possibility that Krystal met with foul play because of the intimate relationship she shared with a man," the detective said.
"This relationship and the subsequent pregnancy may have caused a confrontation between Krystal and this man leading to her disappearance and death."
"It is monstrous, it is despicable, and whoever did this has the most selfish of motivations to look after themselves."
Investigators say Ms Fraser received a 40-second phone call on her mobile from a public phone at 11.59pm on the day she vanished.
Krystal received a 15 calls from the same public phone between May 2009 and the day she went missing.
Almost three hours later her phone was pinged by a tower in Leitchville, 27km from Pyramid Hill and 10 minutes from Cohuna.
The proximity to Cohuna is important because Ms Fraser told several people she planned to attend a party there.
"I believe this caller holds the answers to what happened to Krystal and may be the father of Krystal's unborn child," Det Insp Horan said.
"Investigators strongly believe that the caller was the last person to speak to Krystal prior to her disappearance."
Police have not found Ms Fraser's mobile phone.
Det Insp Horan hopes the increased reward, formerly $100,000, will encourage someone to come forward.
Australian Associated Press