The New Jersey accountant charged in the fatal knifing of his co-worker hired an attorney days after the murder to find out more about the investigation, raising initial red flags to authorities, law enforcement sources told The Post Friday.

Before 30-year-old Kenneth Saal was even considered a suspect in the June 10 slaying of Carolyn Byington, 26, he retained the attorney, sources said.

Employees, including Saal, at marketing firm Engine Group in Princeton were interviewed by investigators after Byington’s slaying and before Saal got a lawyer to ask police further questions about the case, according to the sources.

Saal was arrested at his home in Lindenwold, Camden County, on Wednesday and charged with first-degree murder and possession of a weapon for an unlawful purpose in the death of Byington.

He is accused of stabbing Byinton multiple times and bludgeoning her inside her apartment in Plainsboro while they were on a lunch break.

Byinton, a market research project manager at Engine, was found dead in her apartment after cops did a wellness check at the request of her co-workers.

Before Saal was busted he had asked colleague’s questions about whether people get arrested based on “circumstantial evidence,” court papers stated.

“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.”

Meanwhile, Saal made a brief court appearance Friday at Middlesex County Superior Court where he entered a plea of not guilty in the case, his lawyer said.

Saal’s detention hearing was adjourned until Tuesday. He remains held at the Middlesex County jail.

A lawyer for Saal, Matthew Teeter, told reporters after court that his client is “cloaked in a presumption of innocence until he’s proven guilty beyond a reasonable doubt.”

“I believe that often people in this country lose sight of that and so I would ask that the public respect the family of the victim, respect the Saal family, let the process play out, but don’t forget and definitely don’t lose sight of the fact that he is cloaked in a presumption of innocence until he’s proven guilty,” Teeter said.