'You better kill him': The chilling calls made by slain ...

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An undercover agent who posed as the brother of a Florida professor's alleged killer got much more than he bargained for when he confronted the victim's mother-in-law as part of a sting operation.

The unidentified law enforcement member was investigating the murder of Daniel Markel and looking to confirm a connection between alleged hit man Luis Rivera and Markel's former mother-in-law, Donna Adelson.

Markel was shot dead in July 2014 while in his garage. Rivera and Sigfredo Garcia were later charged with the murder, but Markel's in-laws are suspected of playing a role in arranging the killing in response to his recent contentious divorce from their daughter and sister Wendi.

To investigate the role the Adelsons may have played in Markel's death, the agent approached Donna on the street and said that his brother was in need of money, and asked her for $5,000 to help out the man.

Donna appears shocked and taken aback in video of the interaction which can be seen for the fist time ever on 20/20 Friday night, but not as shocked as police after what happened next.

Authorities allege that soon after Donna met the agent claiming to be Rivera's brother, her son Charlie called his then-girlfriend Katherine Magbanua and said to her: 'You better kill him, because he’s going to be a big problem... if you can't do it I'll have someone else do it.'

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Never-before-soon: New footage reveals shocking details about the murder of David Markel on 20/20 Friday night (Markel's mother-in-law Donna Adelson above with an undercover agent)

Never-before-soon: New footage reveals shocking details about the murder of David Markel on 20/20 Friday night (Markel's mother-in-law Donna Adelson above with an undercover agent)

Suspect?: Police believed that some of Markel's in-laws were involved in his murder (Donna above)

Suspect?: Police believed that some of Markel's in-laws were involved in his murder (Donna above)

Awful: Markel was shot dead in July 2014 outside his Florida home (above with his two sons)

Awful: Markel was shot dead in July 2014 outside his Florida home (above with his two sons)

Video of the encounter between Donna and the undercover agent shows the man approaching her on the street and saying: '“I wanted to let you know that my brother… he helped your family with this problem your family had up north. He’s going through some rough times and I want to make sure that you take care of what he’s going through.'

A stunned Donna responds to this by saying: 'You’re scaring me. I don’t know what you’re talking about.'

That is when the agent hands over the envelope with his request for money and Donna walks away.

In another phone call made shortly after the sting, police allege that Charlie told his mother she should just pay the money as 'a charity.'

Magbanua also reached out to Garcia after the sting according to police. 

ABC News Corespondent Matt Gutman spoke to DailyMail.com about the case ahead of Friday night's episode of 20/20, and revealed how Rivera may come to play a big role in the case.

'State sources tell us that they are in negotiations with Rivera for a plea deal in exchange for his cooperation,' said Gutman. 

That plea deal could have something to do with the ongoing investigation into some members of the Adelson family, and whether or not they paid Garcia and Rivera to carry out Markel's murder. 

The Tallahassee Police Department released probable cause affidavits in the case earlier this month which they believe provided enough evidence to warrant the arrest of Donna and Charlie for the murder of Markel, along with Magbanua.

In addition to being Charlie's girlfriend at the time of Markel's murder, Magbanua is also the mother of Garcia's two children. 

Happier times: Markel and his ex-wife Wendi Adelson (above) had just gone through a contentious divorce and were fighting over custody of their two sons before his death

Happier times: Markel and his ex-wife Wendi Adelson (above) had just gone through a contentious divorce and were fighting over

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