The murder case against Jill Biden’s ex-husband remains shrouded in mystery — including over the motive and even cause of death involving his alleged victim, his second wife.

William “Bill” Stevenson — a famous local former bar owner who was married to the former first lady in the 1970s — on Tuesday appeared briefly by video in New Castle County Superior Court in Delaware, where he pleaded not guilty in the bombshell criminal case accusing him of killing wife Linda Stevenson in December.

Bill, 77, appearing from the jail where he’s being held on $500,000 bail, said, “Thank you, Your Honor,” after the judge explained basic court logistics, including that defendant’s case would be handled by a different judge — and wished him good luck, according to People magazine.

At the end of the brief hearing, a correction officer told Bill it was over.

“OK, cool,” responded the accused murderer, who once owned the nationally known Stone Balloon bar and event space in Newark, Del., where the University of Delaware is based.

Bill’s lawyer had entered the not-guilty plea on his client’s behalf and requested a speedy trial during the hearing. The lawyer, a public defender, did not immediately return a request for comment Tuesday.

Very few details about the slay case have been made public since Bill was arrested more than two weeks ago, including about how Linda was killed and what might have motivated him to allegedly take the life of his wife of over 30 years.

Cops responded to a domestic-dispute call Dec. 28 at the pair’s Elsmere home, finding Linda unresponsive on the living room floor.

Bill wasn’t cuffed until after a weeks-long investigation into her death.

To date, only a one-sentence indictment charging Bill with first-degree murder is publicly available in the case.

He’s due back in court March 16.

Linda’s daughter, Christine Vettori, blasted Bill on social media shortly after his arrest saying: “He killed my mother and justice will be served.

“He’s going to need all the prayers he can get from prison,” the 45-year-old daughter wrote.

In a separate post, Vettori said she and Linda were very close and that her death was devastating.

“The pain of losing her is paralyzing and the emptiness in my heart is an abyss,” Vettori wrote. “Most days I can barely breathe.”

Jill and Bill were married from 1970 to 1975. She hasn’t commented on the allegations against her ex.

Bill has claimed he introduced Jill to former President Joe Biden in 1972 and claimed she cheated on him with Biden — sometime after Biden’s first run for Senate. 

But the Bidens have maintained they met on a blind date in 1975 after Jill’s divorce from Bill was finalized and after Joe’s wife, Neilia, died in a car crash with their daughter.

With Post Wires