Once an engineer with a bright future, Martinez murder suspect’s life collapsed over 18 months

MARTINEZ — Before he allegedly butchered a 68-year-old man during a home burglary at a quiet cul-de-sac, Marcos Iriarte-Valdez likely had hundreds of thousands of dollars soon coming his way.

Until 2025, Iriarte-Valdez, 36, had a life many would envy. He has spent the past decade working as an engineer, for big-name companies like Boeing and Ametek, after earning a degree at California Polytechnic State University in San Luis Obispo. He has lived in some of Contra Costa’s most affluent cities, Orinda and Lafayette, and more recently listed his Martinez townhouse for sale with a $495,000 price tag, according to public records.

But over the past 18 months, Iriarte-Valdez’s life has utterly collapsed into disarray. It started with allegations of domestic violence in early 2025, brought about during an intense breakup and accusations he was stockpiling lewd videos of teens on his computer. By 2026, Iriarte-Valdez was an alleged home burglar, charged with breaking into upscale residences in cities he once called home.

Now, he’s in jail being held without bail, accused of using a knife to stab and kill 68-year-old Todd Stewart. Like Iriarte-Valdez, Stewart had made a career as an engineer. In retirement, he was spending time with loved ones and enjoying leisurely activities like golf and fishing, neighbors told ABC7. On Aug. 6, he allegedly interrupted Iriarte-Valdez during a brazen daytime burglary, and was killed within seconds.

“Everybody is still in shock,” one neighbor told the news station. “I’ve known him for so long. Todd was really a wonderful neighbor, nice guy, always helpful.”

Court records offer a disturbing picture of how Iriarte-Valdez went from a respected professional with a bright future to an accused killer. His case history is also rife with second chances that Iriarte-Valdez received after being accused of serious crimes, including the final one — just a day before the homicide, he’d been released from jail in a still-pending felony burglary case.

It all started Jan. 9, 2025, when Iriarte-Valdez’s ex-girlfriend reported him for alleged domestic violence. The two had recently broken up and he returned to their once-shared apartment to pick up his belongings, in Lafayette. She told police she was working on her computer when he slammed the laptop shut and grabbed her by the throat, choking and scratching her. The attack stop after she managed to grab a stun gun and threaten him with it, authorities allege.

But during the police interview, the woman revealed why they’d broken up, and the investigation drastically changed. She said she’d found videos on Iriarte-Valdez’s laptop of teen girls being sexually abused, including categories of “16 and up” and “17+,” according to court records.

Iriarte-Valdez brushed off the accusations, telling her that pornography sites misrepresent subjects’ ages, but she ended the relationship, according to police. Authorities filed a search warrant for the iPad but weren’t able to locate it, according to court records.

The woman took out a restraining order against Iriarte-Valdez. In April 2025, he was accused of violating it and charged with a misdemeanor. In a hearing before Judge Jesse Hsieh, Iriarte-Valdez was allowed to resolve his case with a diversion program, giving him a chance to avoid a criminal conviction. The diversion program was supposed to end in February 2027, according to court records.

The same month the misdemeanor charges was filed, Iriarte-Valdez’s employment ended with Ametek, an international conglomerate where he’d been working as an engineer, according to his Linkedin page. The page doesn’t show any more recent employers, but Iriarte-Valdez appeared hopeful.

“I’m looking to grow and learn as much as possible with the next step in my career,” he wrote in his profile.

But by May 2026, Iriarte-Valdez was back in legal hot water. This time he was charged with felonies for allegedly breaking into two Orinda homes last April. The burglaries were committed in the middle of the afternoon and Iriarte-Valdez was quickly identified as a suspect and arrested, authorities said.

Over the next three months, two different Contra Costa judges — signed off on his release, likely due to a recent California Supreme Court ruling that has changed how bail is set statewide. Issued April 30, the so-called Kowalczyk decision, requires defendants to be released or “affordable” bail to be set unless prosecutors can show “clear and convincing” evidence the defendant is likely to seriously hurt someone or skip out on court. Proponents of the decisions say its in line with Constitutional mandates, while police and prosecutors have largely decried it as a public safety threat.

Judge Jesse Hsieh initially released him back in May, but placed him on a home detention requirement. In August, he was accused of violating the court’s order and brought before Judge Nichelle Holmes, a former Contra Costa prosecutor, who released Iriarte-Valdez on Aug. 5. The very next day, he allegedly showed up to Francis Court in Martinez and again attempted a daytime home burglary, resulting in the confrontation and Stewart’s violent death, according to prosecutors.

Authorities allege that in addition to video evidence placing Iriarte-Valdez there, his DNA was found at the homicide scene. He remains jailed and is set to be arraigned Wednesday afternoon, records show. Meanwhile, the U.S. Department of Homeland security has publicly commented about his case, in social media posts calling Iriarte-Valdez a criminal “alien” who should be deported.