Police have identified the suspect as Aoki Masanori, the son of Nakano City assembly speaker Aoki Masamichi.
Police received a report at around 4:30 p.m. on Thursday saying a man had stabbed a woman. When officers arrived at the scene, the man opened fire on them with what appeared to be a hunting rifle. The woman and two police officers died. Another woman found later lying on the ground at the scene was confirmed dead.
After the killings, Aoki barricaded himself in the house where he lives with his parents and an aunt. Police say he was armed with a rifle and the standoff lasted for about 12 hours. When he emerged from the house at around 4:30 a.m. on Friday, they detained him.
Police say Aoki has confessed to killing one of the officers, saying, "It's true that I shot him. I killed him."
Investigative sources say Aoki told police he carried out the knife attack because he thought he was being spoken ill of, and he shot the officers because he thought they might shoot him.
Nagano Police chief: 'Extremely vicious crime'
Nagano Prefectural Police chief Koyama Iwao has given a news conference in which he called it an "extremely vicious crime." He said the suspect has confessed, but gave no further details.
A police official at the news conference said officers assumed they were responding to a knife attack, but it turned so quickly into a shooting that they had no time to prepare, and the officers were not wearing bulletproof vests when they were shot.
Koyama says the suspect's mother and aunt managed to escape from the house.
Police officials say the suspect did not make any requests to the police while he was barricaded in the house. They say they persuaded him to surrender with his hands up.
One neighbor says the murdered women were good friends, and often strolled around together.
The suspect's father has resigned from the city assembly.
The police say the suspect received a permit to keep a shotgun in 2015, and for other types of guns later. He currently has permits to possess four firearms.
Gun ownership numbers are low in Japan and gun crime is rare. Last year, there were just nine gun-related incidents reported nationwide. The incidents resulted in four deaths.
Neighbor: Suspect 'an ordinary person'
A man who lives near the Aoki family home says he saw the son emerge from the home silently, carrying a long gun, at around 4 p.m. on Thursday. The neighbor describes him as wearing camouflage clothing, the red vest of a hunting club, and sunglasses.
The man says he remembers speaking to the police about five years ago in connection with Aoki's hunting gun license. He says they wanted to know what kind of person Aoki was. The man remembers telling them Aoki was "an ordinary person."
The neighbor says Aoki was active in his childhood, but as he grew up, he became a person of few words and did not greet people like before.
The man says he has never heard of any trouble with the Aoki family. He says the son was earnestly engaged in farming grapes, and he wonders why that quiet person did this kind of thing.
Suspect had permit to keep guns
National Public Safety Commission Chairperson Tani Koichi told reporters on Friday morning that the suspect had approval from the Nagano Prefectural Public Safety Commission to keep hunting rifles and air guns. He says there were no issues raised during the approval process, and Aoki's permit has been renewed annually.
Sleepless night for locals
At the request of the police, Nakano City turned the gymnasium of a local junior high school into an evacuation center on Thursday night. City officials prepared blankets, food and drinks for those wishing to stay there.
By the time the suspect was detained early Friday morning, 82 locals were taking shelter there.
A man in his 70s said he was scared because he thought people in the area might be in serious danger. He said he spent the night hoping the culprit would be arrested soon.
A man in his 50s said it is painful and regrettable that such an incident occurred to his neighbors. He said that he couldn't sleep at night, and now feels relieved.