'Making a Murderer' investigator opens up on case's unanswered questions

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New book counters assertions in 'Making a Murderer'

Michael Griesbach, a Wisconsin prosecutor who helped exonerate Steven Avery in 2003, is author of 'Indefensible'

One of the investigators at the center of the “Making a Murderer” case revealed that he still has questions about the murder that transfixed much of the country.

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Steven Avery, 54, and his 27-year-old nephew, Brendan Dassey, were sentenced to life in prison in 2007 in connection with the murder of photographer Theresa Halbach in Manitowoc County, Wis. The internationally successful 2015 Netflix docu-series “Making a Murderer” fueled outrage within followers of the case who said police coerced a confession out of Dassey which ultimately led to a conviction.

“Are there parts of it that [Dassey] may have not done? I don’t know. You know, I just don’t know,” the investigator, Tom Fassbender, told NBC News for a “Dateline” special set to air Friday. Fassbender said he “finally realized someone had to bring forward the truth” – even if he couldn’t solve the mystery entirely.

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“Those officers wanted that information in the worst way, and they got it in the worst way. By feeding it straight to Brendan Dassey,” said Laura Nirider, an attorney at Northwestern University’s Center on the Wrongful Conviction of Youth.

A federal magistrate judge overturned Dassey's conviction last August, ruling investigators took advantage of the then-16-year-old Dassey's cognitive disabilities and tricked him into confessing with false promises that he would be all right. The state Dept. of Justice has appealed; Dassey remains in prison pending the outcome.

Just after the “Dateline” segment airs, former Wisconsin District Attorney Ken Kratz’s new novel “Avery: The Case Against Steven Avery and What 'Making a Murderer' Gets Wrong” is set for release. Kratz was the D.A. on the case that got Avery and Dassey prosecuted.

Kratz claimed the docu-series failed to include crucial information. He “systematically erases the uncertainties introduced by the Netflix series, confirming, once and for all, that Steven Avery is guilty of the murder of Teresa Halbach,” according to his book’s description on Amazon.

The Avery legal team thinks differently, as revealed in a “Dateline” preview. When attorney Kathleen Zellner was asked if she thought she had new evidence that could free Steven Avery, she responded confidently, “we do.”

The Associated Press contributed to this report.