SAN JOSE — A spike in juvenile crime in the South Bay last year spurred some local officials, led by the San Jose police chief, to wonder aloud whether Juvenile Hall was being seen as a revolving door by young people committing serious and violent crimes, contributing to higher numbers in assaults, carjackings, and home burglaries.

The notion frustrated probation officials and juvenile justice advocates, who saw the rise as a cyclical outlier, and fiercely contested the idea that dangerous people were being routinely turned away. They cautioned against an impulse for more youth incarceration in defiance of research-supported intervention and diversion programs credited with driving down recidivism and overall juvenile crime in the past five years.

“We all should be concerned by an upswing in young kids committing crimes,” said Laura Garnette, chief probation officer for Santa Clara County. “If I believed there was a shred of evidence that locking them up would ensure they don’t do it again, we would. But incarceration is the single-biggest predictor of future criminality. If you know that, it’s irresponsible to not respond to that.”

There is no question the numbers are up. Based on Santa Clara County cases filed by the District Attorney’s Office, home burglaries attributed to underage suspects rose by 128 percent, from 81 to 185 between 2016 and 2017. It should be noted that 2016 was an unusually low benchmark for this category since 126 cases were filed in 2015.

Robberies were up 21 percent in 2017, from 99 to 120; carjackings rose 250 percent, from eight to 28 cases; assaults increased 59 percent from 32 to 51, and car burglaries jumped 51 percent from 138 to 208.

Figures from the San Jose Police Department, which has by far the largest population jurisdiction the county, show that juvenile robbery arrests rose in 2017 by 53 percent, from 108 to 166. Burglary arrests with the same demographic were up by 51 percent, from 94 to 142; auto theft arrests rose 104 percent from 67 to 137; aggravated assaults rose 5.8 percent from 69 to 73.

The only juvenile crime category that decreased last year was larceny, where juvenile arrests dipped 21 percent from 200 to 157.

Other figures from the county also push back against the idea that Juvenile Hall has become a revolving door: Going back at least three years, the detention rate for arrested minors was between 85 and 89 percent.

As is the case with most serious street crime, a small number of offenders account for a disproportionate amount of incidents, but that doesn’t necessarily mean the same youths are offending over and over again.

“Often it’s the same crimes, with different kids,” Garnette said.

Some recent high-profile cases fueled the discussion. An October robbery crew made up of juvenile suspects were implicated in a 12-hour crime spree throughout the city, and last month a group of mostly teens were arrested in a months-long string of carjackings. But none of those grabbed headlines like the November arrest of a burglary crew that featured a purported 11-year-old getaway driver.

Since his comments late last year, San Jose police Chief Eddie Garcia lauded a new level of cooperation between his department and probation, but also said he still wants to ensure that crime victims don’t get lost in the discussion over how best to reach and divert youths from criminal paths.

“We don’t want to lock them up and throw away the key. At the same time, we have a community being victimized,” he said. “As we work through these issues, we have to make sure that they’re always part of the equation.”

The San Jose cases were also part of a new wave of coordinated crimes involving young suspects who organize among themselves without the infrastructure of traditional street gangs that local experts described as “nontraditional groupings.” Some have loose gang affiliations, but San Jose police treat them as gangs regardless.

The subject has spurred tension between probation officials and their rank-and-file. The probation officers’ union Wednesday publicly voiced their opposition to a personnel reallocation they said was tantamount to the “closure” of the department’s juvenile gang unit. Union president Mark Murray said the move dampens the expertise acquired by seasoned probation officers working on gang problems in the county.

“This cannot be sustained by three officers now assigned to a unit with a different focus,” Murray said to the county’s Public Safety and Justice committee Wednesday.

Garnette objected to the idea that her department has lessened its attention on the issue, and described the personnel decision as a reorganization aimed at addressing gang-involved youth in a more multifaceted way.

“An internal decision was made by the Juvenile Services Division Management Team to restructure the Juvenile Gang Unit and move three probation officer positions into a Specialized Programs Unit that offers a higher level of service,” Garnette said in a statement. “This is a prudent decision that will improve our supervision and treatment of gang-related youth, resulting in better public safety than the current model.”

Murray said he was not convinced by his boss’s explanation.

“We are not fooled by this rhetoric, and you shouldn’t be, either,” Murray said to the committee. “The county … should send a clear message to our community, both gang members and law-abiding citizens alike, that we take criminal street gang activity seriously.”

Meanwhile, probation officials, public defenders, and juvenile-justice advocates bristle at the idea of being more aggressive with youth incarceration.

“At this age, the 14 to 24 age group, the brain is going through all this development,” said Dana Bunnett, executive director of Kids in Common and a board member of the county’s Juvenile Justice Commission. “This is a prime time to get a young person on a positive trajectory.”

Bunnett asserted that intervention, including counseling and involvement in pro-social activities like athletics and political clubs are almost always more effective than incarceration, which doesn’t deter those who are already in turmoil in their home lives.

“We have kids struggling with emotional, social and other kinds of needs, and come into our system through two or three different doors. The foster door, the justice door, the mental-health door,” she said. “The way to make the community safe is with evidence, data, and research-based methods. We can’t just react. We have to be thoughtful.”

A prospective state bill is in the works to help along that cause: AB 1488, authored by Richmond Assemblyman Tony Thurmond, would created juvenile transition centers to provide reentry services and transitional housing for youth offenders upon their release from custody. The proposed services would parallel those offered to many adult offenders who leave jail.

“Too often, youth leave juvenile detention and find themselves without resources or sometimes even a home,” Thurmond said in a statement. “The lack of stability and support makes it more likely that youth will be rearrested. This bill creates a pathway for success for teens leaving the juvenile justice system.”

Garnette echoed the sentiment, saying the factors behind a sophisticated problem like juvenile crime require a sophisticated answer.

“What we’re aiming for is long-term,” she said. “Kids didn’t get into this easily, and they won’t get out of it easily. A child that usually comes out of child welfare, and doesn’t have family support, doesn’t have a lot to lose. (Jail) doesn’t scare them.”

But Garnette also said that philosophy doesn’t preclude Juvenile Hall as a tool to keep the public safe. Currently, the county’s facility has an average monthly intake of around 90.

“This doesn’t mean certain people don’t need to be incarcerated. But only those who might re-offend, or might not come to court. If we need 300 (beds), we’ll have 300, and if we need 50, we’ll have 50,” she said. “If we locked them all up, we might all look great, but it wouldn’t do anything great for public safety.”