When Len Kachinsky opened the Facebook page of his law firm on Monday, he noticed a friend request from a name that immediately caught his attention: Brendan Dassey.

It wasn’t an actual request from the Wisconsin attorney’s most notable former client, who at 16 was convicted of the brutal murder of a young photographer, despite claiming his confession was a lie and he was innocent. It was a fake profile – part of what Kachinsky called “electronic harassment” – created in wake of the 10-part Netflix series Making a Murderer, which features the story of Dassey’s case and attempts to raise significant questions about the efficacy of the American judicial system.

“I haven’t answered yet,” said Kachinsky, “but I posted something urging people not to friend that person.”

Inside his 10th-floor corner office in downtown Appleton, Wisconsin, Kachinsky gestured toward the computer mouse and continued: “But you can observe me; I’m deleting that request.”

Amid the intense response Making a Murderer has prompted – from critics deriding the directors for leaving evidence out of the film to an outcry about the uncomfortable interrogations of Dassey – perhaps no character in the saga has drawn as much controversy as Kachinsky, who was removed from Dassey’s case before it went to trial. In the docuseries, he bears a resemblance to Dana Carvey in a Saturday Night Live sketch, but today, after enduring treatment for leukemia, Kachinsky’s defined comb-over has been replaced with a light wispy patch atop his head. (The cancer has since gone into remission.)

Due to the attention the series has received, the 62-year-old attorney said over the last month he has been the subject of a continuing barrage of criticism levied online and by phone. As he spoke, he clutched a Diet Pepsi and occasionally fiddled with a pile of pens on his desk.

“Most of the calls I’ve gotten are from out-of-state area codes or private numbers, he said. “People who just want to leave it anonymous: ‘Fuck you, I hope you die of cancer’ – stuff of that nature. Not direct threats to commit physical harm, but bad wishes. I’m tired of that.”

He added: “I’m just a convenient target.”

Kachinsky was appointed to represent Dassey just days after he first admitted to police that he was an accessory to the 2005 murder of Teresa Halbach, for which Dassey’s uncle, Steven Avery, was also charged.

The story drew immense notoriety for the connection to Avery, who before his arrest was seen as the face of a US judicial system that can unfairly convict someone: for 18 years, Avery was jailed for a rape he didn’t commit. He was eventually exonerated through DNA evidence.

The case against Dassey was built around the teenager’s own word. A quiet introvert with an below-average IQ of 69-73, Dassey confessed to the crime on 1 March 2005, saying he assisted Avery in sexually assaulting Halbach before murdering her – a nearly four-and-a-half-hour episode called into question by many observers. No forensic evidence tied Dassey to the scene.

But, at the time, Kachinsky believed the confession – false or not – was so damning that it would be nearly impossible for a jury to believe otherwise.

“I looked at it very early on in the case – I won’t sit 10 hours for a Netflix film, but I will sit for four and a half hours for a police videotape recording – and I was convinced his overall demeanor and everything else in that recording was going to convince a jury he was guilty,” Kachinsky said on Monday.

At the time he was assigned the case, Kachinsky said he studied the issue of the false confession phenomenon.

Based on his research, Kachinsky said, Dassey “didn’t meet most” criteria of a false confession.

“My opinion, it wasn’t enough; it didn’t look like it was influencing [enough] to just make stuff up,” he said. “And I thought a jury would come to the same conclusion.”

Laura Nirider, co-director of the Center on Wrongful Convictions of Youth at Northwestern University School of Law, was flabbergasted by Kachinsky’s remarks.

“I don’t know what he’s looking at,” she said. “But Brendan’s confession fits the profile of a [false] confession to an absolute T.”

For example, she said, the officers interrogating Dassey appeared to suggest he would receive leniency for his compliance in confessing to the crime, and then fed facts “to get Brendan to say how Ms Halbach died”.

“So I’m shocked, frankly, that attorney Kachinsky suggests – that anyone would suggest – that Brendan’s confession does not fit the profile of a false confession,” she added.

Dassey’s case has only added to the debate over how reliable confessions are in the judicial system. According to the Innocence Project, false confessions have contributed to 63% of 113 cases in which a suspect was wrongly convicted in a homicide.

Dassey attempted to appeal against the decision in his case, but the Wisconsin court of appeals upheld the ruling in 2013. The state’s highest court declined to hear the case, so Dassey’s current attorneys are attempting to petition a federal court for review. A decision is expected sometime this year.

Asked if he believed Dassey received a fair trial, Kachinsky – who, according to the writ of habeas corpus petition, created an “adverse effect” for Dassey at trial “that flowed directly” from his “disloyalty” – said: “As far as I know, I mean, the … Wisconsin supreme court refused to accept his petition for review.”

Kachinsky appeared to think the case would also meet the same fate in federal court.

“The facts haven’t changed. They got the same problem: what’s the link between anything I did and the verdict in the case?” Kachinsky said. “I don’t see one.”

Nirider disagreed.

Kachinsky is correct that the main piece of evidence used against Dassey was the 1 March confession, said Nirider, who is one of Dassey’s current attorneys working on his appeal.

“But there was a second, very important piece of evidence against Brendan at his trial,” she said. “And that was a recorded telephone call between Brendan and his mother” that happened on 13 May 2006.

To assist in the case, Kachinsky hired a private investigator, after Dassey said he wanted to take a polygraph test. He couldn’t find someone to conduct the exam at an appropriate rate, so he searched the internet and came across the name of Michael O’Kelly.

Len Kachinsky in Making A Murderer. Photograph: Netflix

As a hearing approached on a motion to suppress the 1 March testimony, O’Kelly hatched a plan with Kachinsky to meet Dassey and determine whether he would “accept this idea of testifying against Avery”.

If the judge ruled the confession was admissible, Kachinsky said, then perhaps Dassey would be more inclined to try working out a plea deal. But the video of the meeting between O’Kelly and Dassey reveals an encounter that – to a casual observer – could appear like that of a police officer who was trying to elicit a confession. Despite Dassey’s repeated insistence that he didn’t commit the crime, O’Kelly is seen prodding Dassey to “tell the truth” – or risk “spending the rest of your life in prison”.

If he cooperated, O’Kelly continued, then Dassey could have a shot of striking a deal with the prosecutor and perhaps one day “get out and have a family”.

There wasn’t a deal on the table with the prosecutor, however, said Kachinsky. Early in the case, he said, the extent of his communication with special prosecutor Ken Kratz amounted to discussions on “if there was going to be a deal, what would be the requirement of it”.

“Kratz wanted Brendan to testify against Avery just to make it easier,” Kachinsky said. “With the March 1 statement alone, it was Kratz’s opinion that – plus circumstances showing that Brendan was in the area at the time of the offense – that there would be enough evidence that a jury would find Brendan Dassey guilty, easily.”

The attorney said he was “a bit surprised” by O’Kelly’s tactics.

“O’Kelly was a loose cannon … and I think he still is,” Kachinsky said.

He continued: “Some of the stuff he was telling Brendan, like he was looking at life without parole if he did not cooperate. There were some of the things that I’d consider over the line, that if I knew he was doing that, I probably would’ve stopped it. In 20/20 retrospect, I should have never hired or fired O’Kelly.”

Still, O’Kelly goaded Dassey into signing a statement, confessing once again to his role in Halbach’s death. Before Dassey even left the room, the investigator called Kachinsky; the attorney, in turn, immediately arranged for his client to meet with police the next day.

Though police interrogated Dassey the following day, his testimony was contradictory and provided law enforcement officials with virtually nothing. That is, until one officer appeared to suggest that Dassey should call his mother and admit to the crime. And so he did.

“The police got nothing worthwhile out of that interrogation,” attorney Nirider said. “Brendan was saying just what they wanted to hear; the statement he was saying made no sense. So the investigators solved that problem by saying, ‘Brendan, why don’t you call your mother … over the prison telephone and tell her you did this, because if you don’t tell her we’re going to tell her.’”

That phone call, Nirider continued, would have “never existed” if O’Kelly hadn’t received a confession from Dassey.

“So that’s how attorney Kachinsky’s behavior affected trial,” she said. “It led directly to that telephone call between Brendan and his mother … sadly, his actions went on to taint the trial, and that’s why we believe Brendan deserves a new trial.”

Kachinsky was eventually removed from the case for his decision not to appear with Brendan during the 13 May interrogation, a decision he said was made as a result of a “scheduling conflict” that, in hindsight, he regrets.

“I should’ve been there,” he said. “It wasn’t a big conspiracy to help out the state. I had army duty the next Saturday and they wanted to do it on Saturday to get it done.” After Kachinsky was dismissed from the case, the state public defender’s office decertified him for a period of a time – a decision he said he fully agreed with.

But given that he believed the 1 March interrogation was “convincing”, Kachinsky said he believes he would have removed himself as Dassey’s attorney before the case went to trial.

“I made a professional mistake [not attending the 13 May interrogation],” he said, “but the mistake had no effect, frankly, on the case – other than accelerating the process by which I probably would’ve gotten off that case.”

These days, Kachinsky is winding down his caseload and beginning to ease into a semi-retired lifestyle. He has no plans to watch Making a Murderer.

“I don’t get Netflix at home,” he said. “I’ll rely on what people tell me is in it.”

The father of two also still works as a municipal judge in the nearby town of Menasha. An avid runner, he plans to participate in his 87th marathon this April.

By then, he hopes the brouhaha surrounding the case will have died down. Convinced the Wisconsin court’s decision in Dassey’s case was appropriate, Kachinsky leans again on the appellate decision that cleared him of any claim that he did not uphold his duty to give Dassey adequate counsel.

“It’s unfortunate the way things worked out with Dassey,” he said, “but, I think, anything that went wrong when I represented him had absolutely no effect on the trial.”