'Charismatic' murder suspect believes he's a celebrity, records say. Now he's found incompetent to stand trial

PHOENIX - An accused killer once described by Scottsdale police as "charismatic and manipulative" is no longer heading to trial, at least for now, after a judge ruled Matthew Dieringer incompetent to stand trial.

The backstory:

It’s been a little more than a year since 36-year-old Matthew Dieringer was arrested for first-degree murder, accused of beating 67-year-old Frank Quaranta to death in his Scottsdale home. Court records say Quaranta suffered skull fractures, a neck fracture and broken ribs. Quaranta worked at a heat relief center, where he met Dieringer. At the time, Dieringer was homeless.

At the time, Dieringer was on supervised probation after pleading guilty to theft in April 2025. Police say in those cases, Dieringer slammed a woman’s dog to the concrete and 10 minutes later, tried punching a man before stealing his glasses.

Previously, we have reported on Dieringer’s history of animal cruelty in Colorado, where he mutilated and killed his roommate's two dogs in 2020.

Dig deeper:

Now, there are new words to describe Dieringer.

According to a third mental health expert who evaluated him, Dieringer’s behavior has been called disruptive and erratic. The ruling says he believes he’s the target of a conspiracy due to his self-proclaimed celebrity status.

During a Jan. 22, 2026, court hearing, a Dieringer stated, "Here comes the Rule 11. It’s all scripted!"

That court hearing sparked Rule 11 proceedings to determine if Dieringer is competent to stand trial. Arizona law says a defendant is incompetent when mental illness or disability causes the person to not understand the proceedings or be able to help their own defense.

The court’s recent ruling says one mental health expert documented Dieringer’s frustration with his attorney for not validating his "social media influence" as pertinent to his case.

During the court hearing, the defense attorney stated, "At this time Mr. Dieringer won’t even talk to me, so I don’t know if there is anything he would like to address the court about new counsel."

Dieringer replied, "Yeah, I’d like to fire this guy. He’s not trusted to represent me, my house, my name. My house, I mean my last name. I do not trust that guy to represent me in this (expletive) case."

The judge advised, "You would need to file a motion stating all of the reasons you wish to change counsel and then I would set a hearing to address that."

Dieringer responded, "How about I just write a letter to the FBI, and I’ve already done that. I’m waiting for a response, but I’ll take your advice, and I’ll write a motion, and we’ll let your mouth decide what happens because you seem to like entertainment."

What we know:

The court says when he’s not medicated, he is "uncooperative and irrational" with delusional thoughts. In Dieringer’s previous Arizona cases, he was found competent twice. In his current case, two experts say he’s competent to stand trial, but the third diagnosed him with delusional disorder, compulsive personality disorder, narcissistic personality type and paranoid personality style.

What we don't know:

As for when Dieringer could be deemed competent again, a judge stated there’s no clear evidence he will be restored to competency within the standard 15-month timeframe.

What's next:

Dieringer is now in the restoration competency program within Maricopa County Correctional Health Services. His next court hearing is scheduled for Nov. 12.