The sister of Moira Anderson, the schoolgirl who went missing in 1957, said she was sexually assaulted by the man suspected of her sister's murder.
Janet Hart told BBC Scotland that Alexander Gartshore, who died in 2006, attacked her two years after her 11-year-old sister vanished in Coatbridge.
She said police spoke to her after the incident, but nothing more was done.
Ms Hart now lives in Australia, and has come to Scotland to mark the 60th anniversary since Moira disappeared.
She told BBC Scotland she was out during her lunch hour from school when a man called her over to his car.
"He had the bonnet up at the side," she said.
"He told me to hold his dipstick, and under he went and up - groping me.
"I dropped the dipstick and ran."
She said she turned back to note down his registration plate.
Two policewomen came to the school and spoke to Ms Hart, but she said "nothing more was heard".
"Wouldn't you think something would have happened then?" she said.
Afterwards, the school rector spoke to pupils and advised them to go around the community in pairs, she said.
"That was Alexander Gartshore, I identified him, I remembered him so clearly," she said.
"Luckily I was in an area I could get away - it was broad daylight.
"Unfortunately Moira was trapped on his bus."
The last time Moira was seen, she was boarding a local Baxter's bus. But detectives appear to have failed to follow it up properly at the time.
Had they done so, they would have discovered that the bus driver was Alexander Gartshore, who was on bail at the time facing charges of raping his children's babysitter.
Later that year, he was convicted and sentenced to 18 months in prison for the rape of the babysitter.
Ms Hart said Moira was a tomboy who was full of fun, who loved to buy magic tricks and "play fun on everyone".
She said her sister also had a serious side.
"She wanted to be a missionary and she wanted to go to Africa," she said.
"She wanted to give herself to God."
Ms Hart said the Moira Anderson Foundation was her sister's legacy and was continuing to support children who needed help.
"Children now have a voice and someone who will listen," she said.
She was joined at the commemoration by Sandra Brown, the daughter of Alexander Gartshore.
Ms Brown was convinced her father was the killer and campaigned to have him charged.
In 2014, prosecutors took the unusual step of announcing that Mr Gartshore would have faced prosecution for the schoolgirl's murder if he were still alive.
Police have launched a final bid to find the remains of Moira.
Detectives are to examine specific locations in Coatbridge where Moira's body may have been hidden.
Ms Hart said: "I feel that this time that if she is to be found she will be found."