For almost ten years, the controversial murder of a well-known Florida law professor in 2014 revolved around one sordid question: Did his ex-wife’s family, driven by a bitter custody battle, pay hit men to kill him?

On Monday, a jury found that they did, and Charles Adelson, the former brother-in-law of the deceased professor, Dan Markel, was found guilty of first-degree murder, conspiracy to commit murder, and solicitation of murder. He is likely to be sentenced to life in prison.

Mr. Adelson reacted to the verdict by closing his eyes, saying “No” and lowering his head when it was announced at the Leon County Courthouse in Tallahassee, Fla. The jury reviewed the case for just over three hours following eight days of testimony, including that of Mr. Adelson himself, who took the unusual step of testifying in his own defense.

The victim, Mr. Markel, 41, a renowned legal scholar at Florida State University, was shot on July 18, 2014, in his home in an upscale Tallahassee neighborhood. He was found injured by the police and died 14 hours later at a hospital.

Mr. Adelson, a 47-year-old periodontist from Fort Lauderdale, is the fourth person convicted in the murder of Mr. Markel, which has been widely covered in news articles, TV shows, and a podcast over the years.

“It has been over nine years since Danny was brutally murdered in cold blood, and it has taken a tremendous effort to get to this point,” his parents, Ruth Markel and Phil Markel, said in a statement following Monday’s verdict.

According to prosecutors, Mr. Adelson arranged and paid for two men, Sigfredo Garcia and Luis Rivera, a former leader of the North Miami Latin Kings gang, to drive to Tallahassee from Miami and murder Mr. Markel so that his ex-wife, Wendi Adelson, could move to South Florida with the couple’s two young sons. A judge had denied her request to relocate after the divorce.

The murder was allegedly arranged through Katherine Magbanua, Mr. Adelson’s girlfriend at the time, who had two children with Mr. Garcia.

Mr. Rivera pleaded guilty to second-degree murder in 2016. He was sentenced to 19 years in prison and testified against Mr. Garcia, Ms. Magbanua, and Mr. Adelson.

In 2019, a jury convicted Mr. Garcia of first-degree murder and conspiracy to commit murder. He was given a life sentence.

The same jury that convicted Mr. Garcia could not reach a verdict regarding Ms. Magbanua. Prosecutors retried her case last year following delays caused by the coronavirus pandemic.

She was found guilty of first-degree murder, conspiracy to commit murder, and solicitation of murder. She was sentenced to life in prison, with two additional 30-year sentences to be served consecutively with her life term.

Ms. Magbanua, who had previously claimed to be uninvolved in the murder, altered her story during Mr. Adelson’s trial and testified against him. Wearing a purple prison uniform, she stated that he had instructed her to get Mr. Garcia and Mr. Rivera to carry out the hit.

The lead prosecutor, Georgia Cappleman, asked her, “Why tell the truth now?”

“I believe that the truth needed to come out now so that the family can get some type of closure,” Ms. Magbanua said.

Daniel Rashbaum, Mr. Adelson’s defense lawyer, repeatedly called Ms. Magbanua a liar. “I lied in my trials to save myself,” she said.

Mr. Rashbaum did not immediately respond to a request for comment on Monday.

Prosecutors consider Mr. Adelson’s family members — his sister Wendi Adelson and their parents, Donna Adelson and Harvey Adelson — unindicted co-conspirators. None have been charged and all have denied any involvement. Wendi Adelson has testified with limited immunity in all three trials held in the case.

Charles Adelson was arrested last year, nearly eight years after the killing.

In his telling, he was not conspiring to commit murder when he paid Ms. Magbanua about $100,000 and then hired her for a no-show job at the family’s dental practice for $3,000 a month. Instead, he testified, he was the victim of an extortion plot by Mr. Markel’s killers.

The unexpected defense, presented by Mr. Rashbaum for the first time during the trial’s opening statements, went as follows:

Mr. Adelson’s family was considering giving Mr. Markel $1 million to allow Ms. Adelson and their sons to leave Tallahassee. Mr. Garcia and Mr. Rivera discovered the plan, plotted to kill Mr. Markel, and then, via Ms. Magbanua, threatened Mr. Adelson if he didn’t pay them.

“She’s like, ‘Look, if you don’t pay in 48 hours, they will kill you,’” Mr. Adelson testified.

Mr. Adelson never reported the purported extortion to the police.

In 2016, the police tapped the cellphones of Mr. Adelson, his mother Donna Adelson, Ms. Magbanua, and Mr. Garcia. An undercover F.B.I. agent posed as a member of the Latin Kings gang and asked Ms. Adelson for money to compensate the family of Mr. Rivera, who was in prison.

This led to Mr. Adelson and Ms. Magbanua meeting at a restaurant, where an undercover agent recorded parts of their conversation. Mr. Adelson suggested that the person posing as the blackmailer could be paid off or possibly killed, according to the recording.

And he raised the possibility that the blackmailer might be an undercover agent — although he said he was not worried about that.

“If they had any evidence,” he said, “we would have already gone to the airport.”