A Wisconsin inmate has apparently confessed to the killing of Teresa Halbach, whose murder case was documented in the 2015 hit Netflix series "Making a Murderer." But filmmakers say the confession didn't come from Steven Avery or Brendan Dassey, who have been in prison for the crime for more than a decade.

Avery, 57, and his nephew, Dassey, 29, are currently serving life sentences for the murder of 25-year-old Halbach, who was killed on Halloween in 2005 after visiting Avery's property in Wisconsin. Both men maintain their innocence.

Now, the documentary filmmakers behind the upcoming unaffiliated docuseries, "Convicting a Murderer," claim they received a confession during filming. They have yet to release the alleged criminal's identity because law enforcement has not confirmed the confession's legitimacy.

Exclusive: A Wisconsin inmate allegedly confessed to "Making a Murderer" killing of Teresa Halbach, and it's not Steven Avery https://t.co/hSiMU1Cqk9 — Newsweek (@Newsweek) September 23, 2019

"We haven't confirmed the legitimacy of the confession but seeing as it was given by a notable convicted murderer from Wisconsin, we feel responsible to deliver any and all possible evidence to law enforcement and legal teams," director Shawn Rech told Newsweek Monday.

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Rech did confirm that the confession did not come from Avery or Dassey. It allegedly came from another murderer serving time for an unrelated crime.

"Having been in production for 20 months, we've uncovered an unfathomable amount of information and evidence that is leading us to the truth," Rech said. "Our investigation does not end here."

The Manitowoc County Sheriff's Office did not immediately return CBS News' request for comment.

Attorney Kathleen Zellner, who is currently working to exonerate Avery, confirmed the confession on Twitter.

"We received the handwritten confession on Saturday," Zellner tweeted Monday. "It is worthless unless it is corroborated."

We received the handwritten confession on Saturday. It is worthless unless it is corroborated.#MakingAMurderer2 #WorkingOnIt #NotsoFast — Kathleen Zellner (@ZellnerLaw) September 24, 2019

Earlier this month, Zellner's team announced a $100,000 reward for information that could lead to the "arrest and conviction of the real killer of Teresa Halbach."

"Convicting a Murderer" will be a 10-part series that delves deeper into the case made famous by the original "Making a Murderer" series. Both Avery and Dassey say they were framed for the murder following a lawsuit by Avery of a wrongful sexual assault conviction. He served 18 years of a 20-year sentence before DNA evidence proved he did not commit the assault.

"After watching the series, I was angry with law enforcement and even embarrassed as an American because of what appeared to have happened to Steven and Brendan," Rech said. "But after doing a little bit of follow-up research I learned that not only did I not have the whole story, but I was misled by the series. And I'm saying this as a fan, not as an established documentary filmmaker."

Former Calumet County District Attorney Ken Kratz, who handled prosecution in the case, said he was not informed of the confession.

"To be clear, like everyone else, this is news to me," he tweeted Monday. "I have NO COMMENT until I see the details."