A New Jersey accountant has been charged in the fatal stabbing of his co-worker — after asking colleagues questions about whether people get arrested based on “circumstantial evidence,” according to new court papers.

Employees at marketing firm Engine Group in Princeton told investigators they picked up on Kenneth Saal’s odd behavior shortly after Carolyn Byington’s murder on June 10.

“Other coworkers advised detectives that Saal’s demeanor changed after Byington’s death,” according to an affidavit filed Thursday. “Unprompted, Saal asked one coworker if he could [be] arrested based on circumstantial evidence alone.”

The 30-year-old Camden County resident is accused of stabbing Byington, 26, multiple times and bludgeoning her inside her apartment in Plainsboro while they were on a lunch break.

He made his first appearance in New Brunswick criminal court late Thursday afternoon, but the case was adjourned for a detention hearing on Friday.

Court papers do not reveal a motive for the killing.

Byington, a market research project manager at Engine, was found dead in her apartment just before 6 p.m. after police conducted a wellness check at the behest of her coworkers.

Cops said they determined that her time of death “is believed to be during the same time period as Saal’s lunch break.”

Neighbors have previously said they heard faint screaming and the sound of furniture moving coming from Byington’s unit in the Hunter Glen Drive apartment complex.

Saal allegedly tried to cover up his tracks, claiming he was getting work done on his car and telling his supervisor that he’d be returning late from lunch that day, according to the affidavit.

The boss “later learned that he was lying and not at an auto repair shop,” though it’s unclear how the lies were uncovered.

Meanwhile, other colleagues told investigators that they noticed cuts on Saal’s knuckles and hands “in the days immediately after the victim’s death.”

In a June 19 interview with detectives, Saal said “he worked with the victim but had only seen her in passing on the date of her death.”

He wasn’t able to provide to them details of where he was on his long lunch that day.

But DNA found under Byington’s fingernails connected the married father to the scene. His car was also spotted in Plainsboro on the day of the murder.

Saal, of Lindenwold, is charged with first-degree murder and possession of a weapon for an unlawful purpose in the third degree.

He and Byington both worked at Engine for about three years.

Sarah Cordle, the aunt of Saal’s wife, Chelsea Saal-Cordle, said the high school sweethearts married a few years ago and welcomed a baby last year.

“As far as I know they both seem very laid back, mild-tempered,” Cordle told The Post. “It’s shocking, honestly.”

Saal-Cordle could not be reached.

In a statement, Engine said it was cooperating with police.

“Our primary interest is in helping them get justice for Carolyn’s tragic death,” the statement said. “Our thoughts are with her family during this difficult time.”