For several years, the boy had lived next to Susan Spiller in north Minneapolis’ Lind-Bohanon neighborhood. His sister used to walk Spiller’s beloved greyhound when she wasn’t home. Spiller and the then-14-year-old boy would chat occasionally as she worked in her front yard.

Their apparent closeness made his arrest last week in the 2015 slaying of the well-loved community activist all the more shocking.

Police still haven’t said what motivated the suspect, now 18, to allegedly break into Spiller’s small wood-frame home sometime on the morning of July 16, 2015, and kill her before disappearing.

The questions have piled up since authorities’ announcement last week a breakthrough in the case, which had captured the city’s attention with its brutality and seeming randomness. But for nearly four years, her death remained a mystery as initial leads dried up.

Authorities say the suspect came to their attention last month after matching fingerprints taken after an unrelated arrest with prints found at the crime scene. Citing confidentiality requirements, they would not say what other evidence, if any, linked the teen to Spiller’s killing.

Because he was younger than 18 at the time of the incident, the case is being heard in juvenile court, where hearings are closed to the public and media. But prosecutors have signaled they will try to have him certified as an adult. No timetable has been given for a decision in the matter.

The teen, who made his first court appearance June 25, has been held at a juvenile facility since his arrest the day before. The Star Tribune is not naming him because he hasn’t been charged as an adult.

Authorities have released few details about how the attack played out.

Police arrived to a grisly scene after responding to a report of a break-in at the home in the 5100 block of N. Dupont Avenue. Early reports suggested that Spiller had been stabbed or beaten so badly that medical examiners were unable to determine the exact cause of death. An autopsy instead concluded that she had died of “complex violence.”

Detectives interviewed and eliminated a number of suspects as they pursued the case.

Investigators briefly considered a teenager with a long history of run-ins with the law. A few months after the murder, they were approached by a group of North Side ministers who were convinced he may have been involved and said they could convince the teen to turn himself in. But detectives never located him, possibly because he left the state.

Neighbors also told police to look at a confrontation between Spiller and a group of area kids the day before her death, which left her shaken and fearful. The kids apparently threatened her after she called 911 on them for opening a fire hydrant near her house. A fire truck and crew were dispatched to the scene about 8:30 p.m. that evening to shut off the hydrant, according to a fire report.

Days after the suspect’s arrest, the exact nature of his relationship with Spiller, 68, was still coming into focus, though friends say Spiller doted on neighbors young and old. For several years, the teen lived with his family in the house next to Spiller’s; some months after the attack, they moved to a northwest suburb.

It is not clear whether he was ever seriously considered a suspect at the time.

Then, police caught a break when, on June 12, the teen was arrested in an incident in which he allegedly pulled a gun on someone in the area of N. 41st and Bryant avenues. Officers found him at a nearby gas station, carrying what turned out to be a pellet gun, according to court records.

The records show he was eventually charged with making terroristic threats, a felony offense that required him to provide fingerprints, which were run through a statewide database. Police say a match came back for prints left behind at Spiller’s home, tying him to the slaying.

They did not say whether any DNA evidence was found at the scene.

When reached by phone, the suspect’s attorney, Michael Padden, said he was still waiting to receive discovery materials in the case from prosecutors, but otherwise declined to comment.

The charges come amid an ongoing national debate about how to handle young offenders, which has reached the Supreme Court several times in recent years, according to Barry Feld, professor emeritus at the University of Minnesota Law School and a leading expert on juvenile justice.

In a 2012 landmark decision, Miller v. Alabama, the high court struck down mandatory life sentences without parole for juveniles, based in part on research showing that young brains are still largely undeveloped, he said.

“Basically what the research found, and what the Supreme Court relied on, was the idea that kids are more impulsive than adults. For neurobiological reasons, they are less capable of exercising self-control than adults,” Feld said.

Under state law, prosecutors can petition to transfer murder, rape and robbery defendants 16 and older to adult court, where sentences are often tougher, he said.

But, in cases involving offenders younger than 16, the burden is on prosecutors to show that “based on the nature of the offense and their prior record and their prior exposure to treatment in the juvenile system,” the case should be transferred to criminal court, Feld said.

A third option, he said, is the state’s extended juvenile jurisdiction (EJJ) statute, which gives prosecutors and judges more latitude in dealing with juveniles who commit serious crimes by keeping them under supervision of juvenile court until they are 21.

The focus of the juvenile system is less punitive, Feld said, instead emphasizing rehabilitation — by offering incarcerated youth access to drug and mental health treatment. Juvenile criminal records are also sealed, he added.

A police spokeswoman said in an e-mail this week that the department would not comment on the case beyond what it said at a news conference last month, referring further questions to the County Attorney’s Office, which declined to comment.

In another recent murder case involving juveniles, two teenagers, ages 15 and 16, were charged in connection with the carjacking slaying of a 39-year-old man in northeast Minneapolis last month.