Officials believe Specialist Aaron Robinson bludgeoned Specialist Vanessa Guillén to death. He later killed himself.
Despite a high-profile clemency campaign by allies of President Trump, the administration executed Brandon Bernard by lethal injection for his role in the 1999 crime.
More than a dozen Army officials have been fired or suspended as part of an investigation into the culture at Fort Hood in Texas. Specialist Vanessa Guillen was killed this year after reporting harassment.
The determination means the slain Army specialist’s family is entitled to military life insurance, final pay and allowance.
Christopher Andre Vialva, 40, scheduled for execution on Thursday, would be the seventh federal inmate and the first Black man put to death since federal executions resumed last year.
More Fort Hood soldiers have died in homicides than in battle since 2016, and officials and family members are struggling to understand why.
Army Secretary Ryan D. McCarthy asked those leading the inquiry to examine whether commanders at the Texas base allowed a climate of harassment to spread.
The police said there was no sign of foul play after they found a body, believed to belong to Sgt. Elder N. Fernandes, about 30 miles from the base in Central Texas.
The family of the Army specialist, whose remains were recovered after being reported missing for months from Fort Hood in Texas, remembered her on Friday at a service at her high school.
Remains found in Texas were confirmed to belong to Specialist Vanessa Guillen, who went missing at Fort Hood more than two months ago. Her family said she had dreamed of being in the military since childhood.
In a complaint, prosecutors said that another soldier killed Private Guillen, and that he and his girlfriend tried to dispose of her remains near the Leon River in Texas.
A second suspect killed himself on Tuesday, the Army said. The family of Pfc. Vanessa Guillen called for a congressional investigation into her disappearance in April.
The mystery surrounding what happened to Pfc. Vanessa Guillen, 20, has drawn widespread attention.
A debate is unfolding over whether to rename the installations, as part of a broader national reckoning over buildings, monuments and memorials to men who fought to preserve slavery and uphold white supremacy.