Bill Cosby acknowledged to police more than a decade ago that he fondled Andrea Constand after giving her what he said were cold-and-allergy pills meant to help her relax, his sexual assault trial has heard.

But Cosby also told police that Ms Constand showed no ill effects from the Benadryl tablets and never objected to his behaviour.

The TV star said they been romantic before.

Cosby, 79, is charged with aggravated indecent assault and could spend the rest of his life in prison if convicted.

He has said that the sexual encounter at his Philadelphia home in 2004 was consensual.

The January 2005 interview with police was conducted at his lawyer's offices in New York City about a year after the alleged assault.

Asked by police if he ever had sex with Constand, Cosby replied: "Never asleep or awake."

"I never intended to have sexual intercourse, like naked bodies, with Andrea. We are fully clothed. We are petting. I enjoyed it. And then I stopped, and I went up to bed," he said, according to the statement read to the jury.

Prosecutors decided against charging Cosby at the time, shutting down a police investigation after just four weeks.

A sergeant testified on Thursday that District Attorney Bruce Castor closed the probe hours after police met to review their next steps.

"We had been discussing investigative leads and where they were going," Sgt Richard Schaffer of the Cheltenham Police Department told the jury on day four of the comedian's trial.

Mr Castor issued a press release in early 2005, saying Cosby would not be charged because the evidence had shown both parties "could be held in less than a flattering light".

He said he was concerned that Ms Constand had stayed in touch with Cosby after the alleged assault and waited a year to call police.

A new set of prosecutors brought charges against Cosby a decade later, after a judge unsealed Cosby's sworn testimony from a lawsuit brought against him by Ms Constand.

In his deposition, he talked about giving pills and alcohol to women he wanted to have sex with.

Ms Constand, 44, of Toronto, directed the women's basketball team at Cosby's alma mater, Temple University, where he was a powerful trustee.

She testified this week that Cosby penetrated her with his fingers against her will after giving her pills that left her so limp and foggy-headed that she was unable to push him away or tell him to stop.

She denied they had a previous romantic relationship and said she had rebuffed his previous sexual advances.

On Thursday, a former neighbour, Purna Rodman Conare, testified that Ms Constand became distant and withdrawn in the months after she said the entertainer drugged and violated her.

Ms Conare said Ms Constand's open, easygoing personality changed drastically in early 2004.

Some 60 women have come forward to say Cosby sexually violated them, but the statute of limitations for prosecution had run out in nearly every case.

Ms Constand's case is the only one in which Cosby has been charged.