A forensic expert at the Soham trial has queried how Holly Wells could drown in Ian Huntley's bath without being forced under water.
Mr Cary said it was unlikely Holly drowned accidentally
Mr Huntley's lawyer said Holly fell into the bath, while Jessica Chapman collapsed after Mr Huntley tried to stop her screaming.
The court has also heard that Mr Huntley was charged with raping a young girl in 1998, but later cleared.
Mr Huntley denies murdering the 10-year-old girls in August 2002.
In a statement to police, Mr Huntley's former girlfriend Maxine Carr said she had initially lied to cover for him, because he had a nervous breakdown over the rape charge.
She said Mr Huntley had not asked her to lie for him.
Dog 'broke bath'
The jury heard that Mr Huntley was living in a bail hostel in Scunthorpe when he was accused of raping a young girl.
But when the case came to court CCTV footage showed him to be in a club at the time of the alleged attack, and he was acquitted.
In the same police interview she said the couple's bath needed to be changed because their dog had broken it.
Ms Carr - who requested the interview because she wanted to change her story - told police the dog weighed eight and a half stones.
She said on 4 August 2002, the day the girls vanished, the dog needed a bath but she scrambled about in the tub and cracked it.
Maxine Carr denies helping an offender and conspiring to pervert the course of justice.
Earlier the court had heard pathologist Nat Cary said it was "unlikely at the least" that Holly had drowned when two people were present.
The girls had gone into Mr Huntley's bathroom so he could help Holly with a nosebleed, Mr Huntley's lawyer Stephen Coward QC told the court on Tuesday.
Dr Cary said there is a tendency for nosebleeds to contaminate the hands and drip on objects, including clothing, but that
Holly's top was not contaminated by blood.
He added that even if a top contaminated by blood went into a tub of water... "in my understanding [the top] should still test positive for blood".
Mr Coward said that Mr Huntley had tried to help by handing Holly some wet tissue, but as he did so he slipped and banged into her.
Mr Huntley could not remember exactly how he banged into her as she sat on the edge of the bath, but he did remember hearing a splash, the court was told.
Dr Cary told the court someone who drowns in the bath is usually intoxicated by alcohol or drugs or is an epileptic.
He said that "in all cases" it is a "solitary affair" and that drowning with a third party present is usually a "forced drowning".
"Why no rescue of Holly? Drowning would take some time, it's not instantaneous.
"The restraint, the covering of the nose and mouth of Jessica prevented her,
I would suggest, from being able to rescue her friend from the bath, if that's
He said "vigorous struggling" would be needed to "smother" Jessica.
"You would have to either force them up against something in order to cover the nose or mouth, perhaps a wall, or put your hand behind the head and smother with the other," he said.
"I find it wholly implausible that Jessica somehow is smothered to death in an upright posture."
The pathologist said that there was not much room in the "very compact" bathroom, with three people in it, for Holly "to slip over backwards and then somehow become immersed into the bathwater."
While the defence suggested Holly fell into 18 inches of water, Dr Cary said the overflow was 11 inches high.
Ian Huntley is accused of murdering Jessica Chapman and Holly Wells
He said: "In my view, unless the overflow is in some way blocked the suggested 18 inches doesn't exist as a realistic possibility.
"If the depth is only 11 inches then the opportunity for the nose and mouth
to become completely immersed simply through a passive accident becomes in my view very unlikely."
Mr Coward suggested to Dr Cary that if someone was unconscious when in a bath the "instinctive reactions" of holding the breath and reaching for air would not exist.
Dr Cary replied: "This is absolutely true, but it would still take a period of time for drowning to take place."
Dr Cary said his lack of findings from his examination of the girls' severely
decomposed bodies meant he could not rule out the possibility that they died in the way the defence outlined.
But he also stressed that the lack of findings meant he could not rule out other possibilities, such as strangulation or sexual assault.
Mr Coward offered Mr Huntley's version of events to the court during Tuesday's evidence as background to his cross examination of Dr Cary, one of the prosecution's main witnesses on forensic evidence. Mr Huntley is yet to give evidence.
On Wednesday, Mr Coward corrected one of his earlier admissions, clarifying that Mr Huntley was not sure of where on Jessica's head, or neck, or face he had placed his hand or hands and that when describing events on Tuesday, he had been too specific.
Mr Huntley denies murdering the girls, who disappeared in August 2002. But he admits conspiring to pervert the course of justice.