The Breakthrough ending explained: Who killed Adnan and Gunilla?

The Breakthrough episode 4, the final episode of the Netflix series, sees John and Per catch the double murderer. Contains spoilers.

"I found someone... he shares 102 centimorgans [unit for measuring genetic linkage] with the murderer. There's another one too... who shares 144 centimorgans." Per tells John he needs a third relative and he will know for sure.

He says he needs more swabbing from a wider age range. John says they can't just swab random people and add it to the database. The other cops says they'll try and get more people to volunteer to be swabbed. John gets a call. The journalist wants her interview now as a rival might scoop her. Per being Per agrees to give the interview as long as the journalist does a swab. Per tells her everyone is connected whether you like it or not.

Meanwhile, Per's daughter has run away from school again. Per says he needs to leave the police station to help his daughter. He asks to work from home. John says it's not protocol, but Per says since when has he followed protocol? John says yes as he doesn’t want Per to lose his family like he has.

A red herring

Per returns home. Meanwhile, the football coach from 16 years before comes to the police station. He says he's sorry for not coming forward. The photofit is still troubling him. He says he suspects Ante, whose full name is Andreas Persson. He produces a photo from 2004 from the football club including Ante. John compares the photo of Ante with the photofit and they are similar. Later John learns they did question him at the time and he refused to give a swab. Ante was since convicted of assault and he lived near the crime scene. The police finally make an arrest.

Meanwhile, Per updates John that he's close to finding the third relative. But John says they’ve made an arrest and the suspect fits the profile perfectly.
John returns to the crime scene and memories of the case flood back, including the little boy’s lifeless hand. John later finds the suspect's DNA doesn’t match. Francy, who works with John, calls Per and says she'll send over the results from the swabs she and her colleagues have done. She tells him that the suspect isn't their man.

The killer is revealed

It turns out the journalist, Stina Eriksson, is a close match. "Can’t believe it", says Per to himself and looks to the heavens. Back in his home office he looks at his board and says "that's it". He tells his wife he can't be wrong and must go through it all again. He's found two names David Nilsson and Stefan Nilsson. He calls John who’s filling his car up. "John, the match is perfect. Two brothers. It must be one of them." He explains to John that the journalist is a cousin of the murderer!

Per returns to the office. John says he thinks the killer is David as he's a loner, while the other brother is a family man. John says he wants to send a tactical unit to get David the next morning. John gets his arrest warrant. The journalist, who seems to be amazingly well-informed, tells John she knows about the tactical unit. He drops in the bombshell that it was down to her DNA they've made a breakthrough. Per heads to the scene of the crime and runs into John. Neither of them can sleep. Per's worried he's got it wrong. John tries to reassure him. In the early hours of the morning, John and his team wait in a car. They get a message that the tactical unit is about to move in. They have their man. In the station, they take a swab from the suspect.

John’s told the amazing news the suspect's DNA is a match. He is the killer. John has his man! John phones Per and tells him. He thanks Per.
John and Francy sit down with the suspect in the interview room. John tells David they strongly suspect him of the double murder of Adnan Abbas and Gunilla Pairson. David confesses! David said he had to kill two people, according to the voices in his head, to find peace.

John tells his team: "This is the most tragic case we have ever worked on, and it took such a long time." But he says it’s time to celebrate the fact that all their hard work has paid off and he cracks open the bubbly.

John knocks at Gunilla's husband’s door and he tells him the good news the killer’s been caught. He thanks John. Saad, the father of the young boy, is also told the news the killer has finally been caught. Later, he embraces his wife and their daughter Maya.

The police go to see David’s brother and break the news.

Later, John and Per see Adnan’s family. They describe their pain and how the killer ruined their lives. But perhaps there is hope for the future. He thanks John for keeping his word. John in turn praises Per. Saad presents John with Adnan’s watch and says he wants him to have it. John is reluctant but he accepts it. Then through the door comes Gunilla. She was born three years after Adnan and they named her after the lady who tried to save Adnan.

John meets up with his son. John says sorry for not being there. He suggests they could start playing tennis together. Henry suggests scuba diving and John says he'd love to. Henry tells his father he read about the case and says congrats for solving it.

Per is giving a lecture about his work. Meanwhile, John returns to the murder scene and looks at Adnan's watch. The journalist arrives and says to Per she wants to write a book about his work. She shows him a photo of her with David, the killer, when they were kids. Per says he's keen on the book idea.