Vanessa Guillen disappeared from Fort Hood, Texas back in April.
Authorities said that 20-year old Aaron David Robinson of Calumet City pulled a gun and shot himself as police were trying to make contact with him on Wednesday.
Lawyer Natalie Khawam said the U.S. Army Criminal Investigations Division told her that Robinson bludgeoned 20-year-old soldier Vanessa Guillen with a hammer at Fort Hood and later dismembered her and buried the remains in the woods.
Khawam said she met with investigators from the U.S. Army Criminal Investigations Division on Wednesday evening and was told the other soldier cleaned up the area where Guillen was killed, placed her body in a container and wheeled her out to his car.
Guillen's family has said that she was a victim of sexual harassment prior to her disappearance, something the Army disputes.
Police said a civilian suspected of helping Robinson dispose of the Guillen's body is under arrest. Cecily Aguilar, 22, has been charged with one count of conspiracy to tamper with evidence.
The Army said Aguilar is the estranged wife of a former soldier who was stationed at Fort Hood.
Robinson allegedly admitted to Aguilar that he transferred the woman's body off the base to a remote site in Bell County.
"Subsequently, Robinson enlisted the help of Aguilar in disposing of the dead female's body," read a news release issued Department of Justice U.S. Attorney's Office - Western District of Texas.
According to the criminal complaint, on June 21, what appeared to be the burned remains of a plastic tote or tough box were found in an area near where Robinson's phone pinged. No remains were found that day.
Around 1 p.m. on June 30, contractors working on a fence near the Leon River discovered what appeared to be human remains. When authorities searched the area, they found scattered human remains that appeared to be placed into a concrete-like substance and buried.
According to investigators, Aguilar admitted that Robinson told her he killed a female soldier by striking her in the head with a hammer multiple times at his arms room while on post on April 22, the same day Guillen went missing.
RELATED: Army officials disputes claims Vanessa Guillen was sexually harassed before her disappearance
The criminal complaint said Robinson then placed Guillen's body in a box and moved the box to a location near the Leon River.
Aguilar told officials that sometime between the evening of April 22 and the early morning of April 23, Robinson picked her up at the gas station where she worked and took her to an area near the Leon River and a bridge. A box with wheels and handles was already at the site.
Aguilar said Robinson walked her over to the woods, opened the box, and she saw a dead woman inside. Aguilar later told investigators she "recognized the deceased whom she helped Robinson mutilate and dispose of" as Guillen.
The complaint said Aguilar and Robinson then used a hatchet or ax and a machete-type knife to dismember the body. They allegedly tried to burn the body, and placed the remains in three separate holes and covered them up.
The remains found in a shallow grave near the Leon River in Bell County have yet to be formally identified by authorities.
Aguilar faces up to 20 years in prison and with a maximum $250,000 fine, if convicted.
She's currently being held at the Bell County jail. Her first court appearance will be in Waco.
The hearing is expected to take place early next week.
In Illinois, people came together Friday to bring Guillen's story to light.
Demonstrators marched down Chicago's North Avenue in front of the Illinois National Guard armory, calling for justice.
Though few, if any, knew the 20-year-old soldier from Houston, they began the demonstrations after Guillen went missing from Fort Hood in late April.
"We deserve justice. We will not stop until we get justice," said demonstrator Eli Morales. "This is our country and we deserve justice."
Chicago protesters joined the League of United Latin American Citizens in calling on people not to enlist in the military until there is justice in Guillen's murder.
While the group is certain they'll hold more of these demonstrations, they are looking for a location for a mural they've commissioned in honor of Vanessa Guillen.
KTRK and The Associated Press contributed to this report.