Mark Alan Spadt was arrested more than two months after the 15-year-old girl was found dead south of Yuma.
YUMA, Ariz. — A 52-year-old Yuma man faces a first-degree murder charge and other felony allegations after investigators linked him to the death of a 15-year-old girl who had been reported missing from San Luis, authorities said.
Mark Alan Spadt was arrested Aug. 13 after what the Yuma County Sheriff’s Office described as extensive investigative work and evidence collection. He was booked into the Yuma County Detention Center on one count of first-degree murder, two counts of sexual conduct with a minor, custodial interference and evidence tampering. The sheriff’s office said the investigation remains active and has not released the girl’s cause or manner of death.
The investigation began June 1, when deputies responded at about 10:08 a.m. to a report of a deceased female in the 11700 block of South Adams Avenue, according to the sheriff’s office. The agency’s Criminal Investigations Bureau took over the case. At first, investigators did not know the girl’s identity or age and asked the public for help identifying her.
Two days later, the sheriff’s office announced that investigators had identified the victim as a 15-year-old girl who had previously been reported missing from San Luis, a city in southwestern Arizona near the U.S.-Mexico border. Authorities said her family had been notified. Family members identified the teenager to Arizona’s Family as Diana Lucia Valle Walle. The sheriff’s public statement did not release her name.
Investigators identified Spadt as a person of interest during the early stages of the case, according to the sheriff’s office. Arizona’s Family later reported that a sheriff’s lieutenant said Spadt had been among the last known people in communication with the teenager. After additional investigative work, authorities said they identified him as a suspect. Officials have not publicly detailed the evidence they say connects Spadt to the girl’s death.
At approximately 9:52 a.m. Aug. 13, Yuma County sheriff’s investigators arrested Spadt in the 11300 block of South Avenue 12E. The U.S. Marshals Service and U.S. Border Patrol assisted with the arrest. No injuries during the arrest were reported in the sheriff’s public statement.
U.S. Marshal for the District of Arizona Van Bayless said cooperation among federal, state and local agencies was important in locating and arresting people accused of serious violent crimes. The Marshals Service assisted Yuma County authorities in taking Spadt into custody after the investigation had continued for more than two months.
The charges against Spadt include allegations involving sexual conduct with a minor, but authorities have not publicly released details about those allegations. The sheriff’s office also has not explained the basis for the custodial interference or evidence tampering charges. The allegations are criminal accusations, and Spadt is presumed innocent unless proven guilty in court.
Reports from Arizona’s Family and Law&Crime said investigators had focused attention on Spadt before his August arrest. Law&Crime, citing local reporting, said investigators had previously searched his Yuma residence while examining his connection to the teenager. The sheriff’s Aug. 13 release did not provide a full chronology of investigative steps or describe what evidence ultimately led authorities to seek his arrest in the death investigation.
The sheriff’s office has continued to withhold details about how the teenager died, saying information concerning her death is not being released while the investigation remains underway. That leaves several central questions unanswered, including the circumstances leading to her death and the specific evidence prosecutors may rely on to support the charges.
Spadt remained accused of first-degree murder and the additional offenses announced by the sheriff’s office following his Aug. 13 arrest. No later court disposition was confirmed in the sources reviewed. The Yuma County Sheriff’s Office said the case remains under investigation.
Author note: Last updated August 16, 2026.