Police in Tasmania are offering rewards of $500,000 for information about seven cold case murders dating back to the early 1990s.
Investigators on Monday announced they were boosting rewards for information that leads to a conviction.
They believe there is "significant, credible information" that Launceston man Christopher Dean Watkins was abducted and murdered.
The 28-year-old was last seen on August 7, 2013, with police previously saying he made a panicked phone call to family after two men visited his home.
"Extensive investigations have been undertaken but no one has been charged in relation to his murder," Tasmania Police said in a statement.
Police initially offered $50,000 for information relating to Mr Watkin's disappearance.
"Families always deserve answers to what happened to their loved ones and we owe it to them to find those answers," Assistant Commissioner Jonathan Higgins said.
"In the passage of time someone's circumstances and allegiances may have changed.
"The offer of a life-changing amount of money may be the motivation they need to come forward. This kind of money allows a fresh start."
OTHER SUSPECTED MURDERS WITH $500,000 REWARDS
* Nancy Grunwaldt. Went missing from the state's east coast in 1993. Extensive investigations have been carried out.
* Victoria Cafasso. Was murdered on Beaumaris Beach on the east coast in 1995. No one has been charged after several lengthy investigations.
* Paul Winston Byrne. Reported missing in 1996. It is strongly suspected he was murdered in Rossarden in northeast Tasmania.
* Helen Munnings. Disappeared from the Burnie area around 23 July, 2008. The coroner determined Helen died in Burnie on or about this day. It is strongly suspected she was murdered.
* Simon Crisp. Shot dead in the carpark of the Marrawah Hotel in the northwest in 2013. After extensive investigations, no one has been charged.
* Eve Askew. Reported missing aged 14 from her home in Fitzgerald in the south in November 1991. It is believed she met with foul play.