Cecily Aguilar, who was initially identified by Texas Rangers as "the estranged wife of a former Fort Hood soldier", helped dismember and bury the body — and was also the motive behind the attack, attorney Natalie Khawam claimed.
In an interview with Nancy Grace on her podcast "Crime Stories", Khawam claimed Guillen was beaten to death with a hammer while on the army base.
She told the host she had met with the US Army Criminal Investigation Command (CID), who revealed gruesome details of the attack.
She claimed Guillen was summoned to the armory at Fort Hood on April 22, the day she disappeared.
Khawam said Guillen discovered Robinson was seeing Aguilar, which was against army regulations, since she was still married to another soldier; when she informed him she had to report him, he replied "No you're not, you're not going to ruin my career," and allegedly grabbed the first thing he could find — a hammer.
The attorney said he bludgeoned her skull over and over, spraying the armory with blood.
Khawam said he stashed her body in a container outside the armory, and went home, before returning back to the base at 6 p.m. that evening.
Taking the body to a nearby river, he then called Aguilar to ask for help burying her, Khawam said.
After attempting and failing to burn Guillen's body, they then used a machete to chop her to pieces, according to the attorney.
After that, they dug a shallow grave, and buried her remains in quick drying cement, the attorney retold; they then destroyed Guillen's cell phone and tossed it, along with the hammer and machete out the window as they drove off.
A picture of that gravesite was released on Wednesday. The remains have yet to be formally identified.
Khawam alleges an army cover up; she questioned how Robinson had managed to completely clean up the murder scene in just 43 minutes, how nobody had heard Guillen scream, or how there was no CCTV footage.
Robinson is said to be the same superior who Guillen had told her family was sexually assaulting her.
The charges against Cecily Aguilar are not specified beyond "third-degree felony"; she was released on bond on Thursday.
In an emotional press conference the day before, Guillén's sister Mayra claimed she had met Robinson on the first day of looking for her sibling at Fort Hood — and he had laughed in her face.
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"They lied to us since day one. When I first went up to that base, that subject, I met him, not knowing that he had something to do with it," she said, trying to contain her emotions.
"I felt... something was telling me that he did something. And I wasn't wrong."
"He still had the nerve that same day to laugh in my face. And apparently now he kills himself. Why I don't know. But whoever is responsible has to pay."