Bernadette Walker, 17, was last seen alive on July 18 last year when 51-year-old Scott Walker, who was not her biological father, collected her from his parents’ house in Peterborough, Cambridge Crown Court heard.
Prosecutors said that Scott Walker killed her to “prevent her pursuing her allegations of sexual abuse any further”.
Her body has not been found despite police searches.
Scott Walker said that Bernadette ran away from his car when he stopped the vehicle, but jurors rejected his account.
The prosecution said Bernadette told her mother, 38-year-old Sarah Walker, on July 16 that Scott Walker had sexually abused her “over a number of years”, but that Sarah Walker did not believe her daughter’s allegations.
Scott Walker told jurors that Bernadette’s allegations of sexual abuse were “untrue”.
The prosecution said that Bernadette was sent to stay with Scott Walker’s parents overnight on July 17 “while things calmed down a little”, with Scott Walker collecting her on July 18, when she was last seen alive.
Sarah Walker reported Bernadette as missing to police in the early hours of July 21.
Lisa Wilding QC, prosecuting, said that Scott Walker formed an “unholy alliance” with Bernadette’s mother, his ex-partner Sarah Walker, to cover up the girl’s death, sending messages from Bernadette’s phone to give the impression she was still alive.
Sarah Walker admitted two counts of perverting the course of justice by sending messages from Bernadette’s phone after she disappeared and by providing false information to the police relating to her disappearance.
Scott Walker told the court that Sarah Walker had feared the involvement of social services over the allegations against him.
Sarah Walker denies two counts of perverting the course of justice “knowing or believing” Bernadette to be dead, and jurors are continuing to deliberate on these.
The judge gave them a majority direction on Monday.
Scott Walker was also found guilty of two counts of perverting the course of justice.
Ms Wilding said that Scott Walker’s phone, “which was usually in regular use”, was off between 11.23am and 12.54pm on July 18.
“The prosecution say that in that hour and a half he killed Bea,” she said.
Ms Wilding said that when Scott Walker’s phone reconnected to the network at 12.54pm the first call he made was to Sarah Walker, which lasted for more than nine minutes.
“The only sensible conclusion that can be drawn from that telephone call is that Scott Walker told his wife that he had killed Bernadette and needed her help, immediately, to cover up Bea’s disappearance and death, and to buy them both time to work out what should happen next,” said Ms Wilding.
“The story they concocted in that call, and which both relied on from that moment on, even until now, was that Scott had stopped the car on the short drive home to confront Bea about her allegations, that Bea jumped out of the car when he pulled over and that she ran off.
“Then, that Scott tried and failed to run after her and so returned home without her.
“From that moment on, Scott and Sarah Walker – Bea’s own mother and father – were joined, the prosecution say, in an unholy alliance, designed and intended to mislead, to divert and to pervert the inevitable investigation into the disappearance and ultimately the death of Bea Walker.”
Sarah Walker was not married to Scott Walker but had changed her surname to Walker by deed poll.
At the time of Bernadette’s disappearance, Scott and Sarah Walker were living at the same address but Sarah Walker was in a relationship with another man.
Scott Walker said in evidence that he considered the possibility that Bernadette’s allegations and disappearance may have been a “plan” to get him “out of the house”.
Sarah Walker did not give evidence.
A sentencing date has been set for September 10.