On March 1, 2006, Dassey, then 16, told police he had helped Avery rape, stab, shoot and dismember Miss Halbach on his uncle's orders.
The confession, which Dassey made during a four-and-a-half hour interview with two seasoned police investigators, was made without a lawyer or his mother present.
The teen has an IQ score of 70, which qualifies him as intellectually disabled.
He later said his confession had been coerced. Both he and Avery claim they are innocent.
No physical evidence or DNA was found linking Dassey to Halbach's murder or to support his claims.
On March 2, Dassey was charged with being a party to first-degree intentional homicide, mutilation of a corpse, and first-degree sexual assault.
On March 8, Len Kachinsky was drafted to represent the boy.
The 16-year-old's case was contracted out to Kachinsky's private practice by the state, a typical scenario when there is a conflict of interest for the public defender or there is an overwhelming caseload.
Making a Murderer suggests that Dassey never had a chance at a fair trial because Kachinsky believed he was guilty from the outset, urging him to take a plea deal and testify against Avery.
Kachinsky told Daily Mail Online that he had not watched Making a Murderer and had no plans to.
However based on the messages he has received from disgruntled fans and reading news articles, he believes that his role in Dassey's case has been misunderstood.
He said: 'I get tired of it after a while because so much of it's based on a misinterpretation and incomplete understanding of what exactly happened in the case, and my role in it regarding Dassey.
'What [viewers] are missing is on March 1, 2006, before he was charged or had an attorney, Dassey gave a four-and-half hour, videotaped confession to the police. This was used at Dassey's trial.
'When I got the case, I read the complaint and soon thereafter, I got the four-and-a-half hour video confession and watched it as quickly as I could.
'I was concerned about [Dassey's age and low IQ] so I made a motion to keep it out of evidence but that was denied.'
On May 12, 2006, Manitowoc County Circuit Court Judge Jerome Fox ruled that the prosecution could use Dassey's March 1 statement as evidence despite Kachinsky's argument that the teen had been coerced.
Following the hearing, Kachinsky said he would look at all his options with his client including making a deal with prosecutors.
He said: 'This is an extremely important decision that a 16-year-old is going to have to make. I can give him advice, but ultimately it's his decision.'
Kachinsky told Daily Mail Online that although he never said so publicly, he thought Dassey had a slim chance of getting off and was trying to do his best for him under the circumstances.
'I never explicitly told anybody what my strategy was but it was pretty obvious I think to some people, that I was trying to get a good deal for Brendan. I didn't think he had much of a chance at trial.'
He added: 'But if Brendan wasn't going to admit to being involved in the murder, I'm not going to encourage him to take any type of plea deal.'
The attorney told Daily Mail Online that he does not make a personal call on whether his client his guilty or innocent but focuses on finding the best option for their defense.
Kachinsky said on Saturday: 'I never make a personal judgement on whether someone did something or not.
'But after I had seen Dassey's confession, the main question for me is to whether the state can prove beyond a reasonable doubt to a jury that somebody did it.
'I had the impression that this videotaped statement [from Dassey] would be extremely convincing and if a jury saw it, it didn't appear to be overly leading or anything else.
'Dassey's demeanor on the video tape... he appeared to understand what was going on, it looked that way to me.
'I thought a jury would come to that conclusion as well.'
After it was clear the jury would see Dassey's confession, Kachinsky drafted in a private investigator, Michael O'Kelly, to try to figure out the truth about the teen's confession.
Kachinsky said to this day he regrets bringing on Mr O'Kelly in the case.
He told Daily Mail Online: 'I got in Michael O'Kelly because I couldn't get anyone else. I had never heard of the guy before, he was a wild card.
'He was from Chicago I think. He becomes my investigator, trying to figure out what's going on.
'After the motion was denied on the Friday, I told him to go talk to Brendan in the afternoon.
'Apparently he videotaped it but that tape was never released to the state or to anyone else.
'That was only for our internal use – we wanted to find out whether Brendan's position was that he agreed with his March 1 statement or with his subsequent denials.
'O'Kelly's statement was never to be used in evidence. I realize he probably went overboard on some of the stuff he did.
Kachinsky added: 'The guy had been to law school for a year or two and frankly I didn't think he needed close supervision but apparently he did. Looking back, I wouldn't have hired O'Kelly.'
O'Kelly's interrogation of Dassey led to another confession to the murder, and the teen even provided him with a stick-person drawing of Miss Halbach apparently chained to Avery's bed.
Kachinsky said: 'I got a call, at quarter to nine on the Friday night [from O'Kelly]. He said Brendan desperately wanted to give a statement to the police.
'I had Army Reserve drill in the morning so I couldn't be there.
'He [O'Kelly] was going to be there – and if anything was to go awry, he'd call me right away and I had my phone with me the whole day.
'That interview didn't go too well for anybody. I saw the recording a week or two later. It wasn't used as evidence at Dassey's trial.'
Kachinsky continued: 'I was personally criticized for not being there [for Dassey's second police interview].
'Perhaps with 20/20 hindsight, that was a valid criticism, I should have postponed the interview. I couldn't postpone the army drill but I could have postponed the interview.'
Kachinsky wanted to clarify what he said were any misconceptions that Making a Murderer may have given about Dassey's interview by O'Kelly.
'I don't know if the show made it clear but it seems a lot of people have the impression that the jury saw the interview that Brendan gave to Bill O'Kelly, and the second interview with police in May, and concluded that led to him being convicted.
'None of the Bill O'Kelly's interview or information was used at trial. None of Dassey's second interview with police investigators was used at trial. The March 1 interview was all that was used at trial.'
Kachinsky added: 'The state's view of the whole thing was, we have this March 1 confession of four and a half hours, we've got him.'
He was taken off Dassey's case in August 2006 for allowing the teen to be interviewed by police in May without an attorney.
Kachinsky stands by his opinion that a plea deal would have been the best option for Dassey.
He believes there was pressure on the teen from the Dassey and Avery families not to testify at Steven Avery's murder trial.
Kachinsky said: 'They were telling him not to cooperate with the state and drop the dime of Uncle Steve.
'It would have given Brendan the better deal, if he was going to testify truthfully against Uncle Steve.
'Helping the prosecution in their case against Avery would have been a by-product of that.'
Kachinsky said he was convinced Dassey's confession would led the jury to find him guilty.
'In some cases you want to fight the state to the last juror. Some cases are damage control and given the strength of evidence against Dassey and the strength of the confession, this was damage control,' he said.
When asked about the other evidence against Dassey, Kachinsky told Daily Mail Online: 'The confession was basically it.
'There was some other evidence that he was home and in the area, he didn't have a good alibi.
'The March 1 confession was pretty detailed.'
Kachinsky denies suggestions made by the documentary and Dassey's current defense team that he was disloyal to his client.
He said: 'If you labelled a defense attorney disloyal every time the client says "I'm innocent" and the attorney says back, well I don't think the evidence is very helpful to us, then every attorney out there has been disloyal at some point.
'There is nothing that I did in the course of the case that made life any more difficult for his subsequent attorneys.'
Kachinsky said that he followed Dassey's trial in the local newspapers. In April 2007, Dassey was found guilty of Teresa Halbach's murder and sexual assault, and sentenced to life in prison with the possibility of early release in 2048.
His former attorney said: 'I wasn't surprised by the verdict or the length of sentence.
'Part of the reason for the length of the sentence I'm sure was that the judge was aware that Dassey knew he had alternatives.
'That's why he got more time than he would have done otherwise had he been willing to testify against Steve.
'Testifying against Steven Avery would have taken a certain amount of personal bravery.'
Had Dassey taken a plea deal, Kachinsky estimates that he could have been released sometime in the 2030s when he is in his early forties.
With his current sentence Dassey will only be considered for parole at 58 years old.
Now 26, he is serving his sentence at Green Bay Correctional Facility in Wisconsin.
A lawsuit for Dassey has been taken to federal court in Wisconsin by Northwestern University's Center on Wrongful Convictions of Youth.
The suit claims that Dassey was illegally imprisoned in 2005 and asks that Dassey be granted a writ of Habeas corpus, meaning his case must be re-examined.
A decision is likely to be made in the next year.
Kachinsky said he had never considered whether Dassey may have been wrongfully imprisoned as Making a Murderer suggests.
He said: 'Personally, I never make any judgement on that. I could understand how the jury came to the verdict they did.'