NORRISTOWN — A suburban Philadelphia judge hopes to decide whether to dismiss a sexual assault case against actor Bill Cosby over an unwritten promise of immunity a former prosecutor says he gave Cosby’s now-deceased lawyer a decade ago.

Judge Steven T. O’Neill puzzled over the testimony of former Montgomery County District Attorney Bruce Castor on Tuesday and peppered him with questions as Castor wrapped up hours of testimony for the defense. The proceedings were to resume today.

Cosby, 78, was arrested and charged in December with drugging and violating former Temple University athletic department employee Andrea Constand at his suburban Philadelphia mansion in 2004. He could get up to 10 years in prison if convicted.

Castor said he found serious flaws in the case in 2005 and declined to bring charges. He said he steered the matter to civil court so Constand could become “a millionaire.” But he also maligned her credibility throughout the day and questioned whether she and her mother set out to extort Cosby.

“If there was an agreement, why didn’t you make that agreement in writing?” the judge asked Castor.

“It was unnecessary because I concluded there was no way the case would get any better,” he said.

Castor suggested that Cosby and then-attorney, Walter Phillips, who died last year, had the same understanding, because Cosby later agreed to testify without invoking his Fifth Amendment right against self-incrimination in a lawsuit brought against him by Constand.

“Cosby would’ve had to have been nuts to say those things if there was any chance he could’ve been prosecuted,” Castor said, referring to the damaging testimony from a deposition unsealed last summer.