Suspect in violent murder of Scottsdale man on probation after prior plea deal

SCOTTSDALE, Ariz. — The man accused of brutally killing 67-year-old Frank Quaranta in a Scottsdale townhome last week has a violent history, raising questions about whether the criminal justice system did enough to prevent the tragedy.

Police said 34-year-old Matthew Dieringer beat Quaranta to death last week. Police said Quaranta was allowing Dieringer to live with him after meeting at a local church where the victim was serving months ago. Dieringer was arrested on Sept. 6, nearly a week after the killing, when he was on the run from law enforcement.

Court records reveal a troubling pattern of violence and instability. Dieringer, who prosecutors said was homeless and struggling with mental illness, had been released from jail just four months ago after accepting a plea deal that put him on probation instead of behind bars.

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During Sunday’s hearing, a prosecutor outlined Dieringer’s criminal past, which began in Colorado, where he served 18 months in prison for animal cruelty after killing and dismembering his roommate’s dog.

After his release, records show Dieringer moved to the Valley. In the fall of 2024, Phoenix police arrested him on three felony charges — robbery and two counts of animal abuse. Investigators accused him of picking up a dog at random and slamming it to the ground in front of its owner. Hours later, he allegedly kicked another dog into traffic.

Months later, surveillance video captured Dieringer attacking a man walking to work in downtown Phoenix. Despite those incidents, prosecutors did not pursue the animal cruelty charges, citing a lack of evidence for conviction.

Former prosecutor at the Maricopa County Attorney’s Office, Josh Kolsrud, reviewed Dieringer’s record and the written evidence Phoenix police collected to obtain probable cause in his prior cases.

“These are pretty egregious cases, and the statistics are also very clear that people who commit animal cruelty acts oftentimes escalate their behavior and commit crimes, assaults against humans later on that are very egregious,” Kolsrud said.

Instead, the Maricopa County Attorney’s Office indicted Dieringer on low-level charges — felony theft and misdemeanor assault. He accepted a plea agreement this spring, pleading guilty to theft in exchange for two years on supervised probation.

RELATED: Scottsdale murder suspect arrested after being spotted by citizen, police say

Dieringer was released from jail in May but never reported to a single probation meeting, including on the exact same day of his release, according to court records. 

Records show his probation officer tried to find Dieringer by phone and by visiting an address listed for him; both attempts were unsuccessful. Within three weeks, probation officers moved to revoke his release, and an arrest warrant was active at the time of Quaranta’s murder.

“They gave him a slap on the wrist, and he got a light sentence,” Kolsrud said. “When his probation was revoked, the judge only gave him a $250 bond, which doesn’t account for the dangerousness of this person. We failed this individual, and we failed our community.”

In Dieringer’s earlier assault case, the victim, who was allegedly attacked at random, pleaded with the court to impose the harshest possible sentence, warning: “Who is to say, if released, Matthew won’t go back on the streets and assault innocent people,” he questioned in an impact statement shared with the judge.

Maricopa County Attorney Rachel Mitchell’s office said prosecutors believed the theft and assault charges were appropriate “based on the circumstances of the crime.” 

A spokesperson added that animal cruelty charges were turned down because of “no reasonable likelihood of conviction. 12News asked why a plea deal was offered to Dieringer in the first place. 

“We cannot discuss specific information regarding the plea agreement,” MCAO said.

Dieringer is now being held in the Maricopa County Jail, facing a first-degree murder charge. He’s being held on a $2 million cash-only bond. 

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