Queensland authorities hope a $500,000 reward will finally expose the person who murdered Mackay teenager Marilyn Wallman 46 years ago.
A long-standing reward of $250,000 was doubled on Tuesday, with detectives saying time is of the essence given how long ago the schoolgirl was snatched and killed.
The 14-year-old was last seen 46 years ago, on March 21, 1972 when she climbed onto her bike for the ride from her Mackay home to school.
A piece of her skull was found in the nearby town of Mirani two years later, but it was only identified as Marilyn's in 2015.
Detective Inspector Damien Hansen said it was "crunch time" for the investigation following a review by Mackay cold case detectives.
"We've got one person of interest ... it has put us in a position now where we're taking this extraordinary step ... in the hope that final straw will come through," he told reporters.
Det Insp Hansen says it was critical an arrest was made soon as witnesses and the suspects weren't getting any younger.
"It's getting more and more difficult, if we were to make an arrest now, by the time it goes through court it may well be 50 years," he said.
Marilyn's family has been tormented by the lack of answers, particularly her brothers who were riding about 10 minutes behind their sister on that fateful day.
As they followed the route she'd taken towards their school, they found her bike and school bag abandoned in the street.
In 2016, her brother Rex Wallman said his family had not given up hope that the killer would be caught.
At the time, police appealed for information about a two-tone aqua and white Holden sedan, believed to be an HD or HR model, that was spotted near where Marilyn vanished.
Police Minister Mark Ryan says this is the first time in Queensland history that a reward has been increased to help solve a murder.
It's also the largest government reward ever offered for a case of this kind, however, the offer expires in 12 months.
"We want to get to the end of this story. We want to provide justice not only for Marilyn but also for her family," he said.
"Her family want to know what happened and who was responsible."