Natalie Khawam announced on Thursday that Guillen, whose body was thought to have been located earlier this week, had been attacked with a hammer in the Fort Hood armoury room where she worked.
US Army investigators told Ms Khawarm’s clients on Wednesday that Guillen’s body was then transported outside the base, reported CNN.
The victim had been due to submit a harassment complaint against Aaron David Robinson, who police named on Thursday as the main suspect in the case, when she went missing on 22 April.
He died by suicide on Wednesday when police tried to communicate with him.
Agent Damon Phelps told a news conference that “while law enforcement agencies attempted to make contact with the suspect in Killeen, Texas, Specialist Robinson displayed a weapon and took his own life”.
A second suspect, Cecily Aguiliar, was said to have been told by Robinson that he had killed another soldier.
Mr Phelps added that Aguilar, the estranged wife of a former Fort Hood soldier, had been enlisted in transporting Guillen’s body from the base,
She faces one count of conspiracy to tamper with evidence in Guillen’s disappearance.
Authorities had announced on Tuesday that human remains were discovered near Fort Hood, but investigators have not confirmed they belong to Guillen.
Mr Phelps said some 300 interviews had been conducted in their investigations, and that there was “no credible information” Guillen had been sexually harassed or assaulted.
Mayra Guillen, reported CNN, said this week that she believes Robinson “did something”.
“When I first went up to that base, that subject, I met him, not knowing that he had something to do with it. I felt something was telling me that he did something, and I wasn’t wrong apparently,” she said on Wednesday. “And apparently now, he kills himself. Why? I don’t know.”
“But whoever is responsible has to pay, and we demand a congressional investigation."