Mark Alan Spadt faces seven felony charges and remains in custody on a $1 million cash-only bond.
YUMA, Ariz. — A 52-year-old Yuma man accused in the death of a 15-year-old girl who had been reported missing from San Luis has been formally charged and remains held at the Yuma County Detention Center on a $1 million cash-only bond, according to local court reporting.
Mark Alan Spadt faces seven felony charges, including first-degree murder, sexual conduct with a minor, custodial interference and evidence tampering. Judge Gregory S. Stewart kept Spadt’s bond at $1 million cash only during a court proceeding reported Aug. 17. Spadt is scheduled to return to court Friday at 4 p.m. The charges are accusations, and Spadt has not been convicted in the case.
The case began June 1, when the Yuma County Sheriff’s Office responded at about 10:08 a.m. to a report of a deceased female in the 11700 block of South Adams Avenue. The agency’s Criminal Investigations Bureau took over the investigation. At the time, authorities did not know the person’s identity or age and asked the public for help identifying her.
On June 3, sheriff’s officials announced that investigators had identified the girl as a previously reported missing 15-year-old from San Luis and said her family had been notified. Family members later publicly identified her as Diana Walle, also reported by Arizona’s Family as Diana Lucia Valle Walle. The Sheriff’s Office did not release the teenager’s name in its Aug. 13 public statement.
Investigators identified Spadt as a person of interest during the early stages of the death investigation, the Sheriff’s Office said. Lt. Sam Pavlak later told Arizona’s Family that Spadt was among the last known people who had been in communication with the teenager. Authorities have not publicly detailed the evidence they say ultimately linked Spadt to her death.
Spadt had also been arrested June 17 on unrelated drug allegations, 16 days after the teenager was found. Law&Crime reported that investigators had searched his Yuma residence while examining his connection to the girl and that he was later released on bond in that case. Those earlier allegations are separate from the charges now filed in connection with the teenager’s death.
After what the Sheriff’s Office described as extensive investigative work and evidence collection, investigators identified Spadt as a suspect. Deputies arrested him at approximately 9:52 a.m. Aug. 13 in the 11300 block of South Avenue 12E. The U.S. Marshals Service and U.S. Border Patrol assisted with the arrest.
The Sheriff’s Office initially said Spadt was booked on one count of first-degree murder, two counts of sexual conduct with a minor, custodial interference and evidence tampering. A later court report said he is now facing seven felony charges. Public reporting available Monday did not provide a complete count-by-count breakdown explaining the difference between the initial booking allegations and the seven formally filed charges.
Authorities have not released the teenager’s cause or manner of death or explained the specific evidence supporting the murder allegation. They also have not publicly provided details behind the sexual conduct, custodial interference or evidence-tampering allegations. The Sheriff’s Office has said the investigation remains active.
Spadt remained in the Yuma County Detention Center as of Aug. 17 with his cash-only bond unchanged at $1 million. His next reported court appearance is scheduled for Friday at 4 p.m. as the criminal case proceeds and investigators continue withholding details about the circumstances of Walle’s death.
Author note: Last updated August 17, 2026.