Garcia made the announcement Friday along with leaders from the League of United Latin American Citizens, or LULAC. The group met with McCarthy earlier in the day to demand the independent investigation.
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McCarthy confirmed the news in a post on Twitter.
"I am directing an independent & comprehensive review of the command climate and culture," McCarthy wrote. "We have to listen in order to create enduring change."
Garcia and LULAC leadership said it would be presenting more cases to the Army, including those of other Latino soldiers who reported sexual harassment and victims of still unresolved homicides and abuses.
Guillen, who had been missing since April 22, was killed and dismembered by U.S. Army Specialist Aaron Robinson, a 20-year-old soldier from Calumet City, Illinois. He took his own life before he could be taken into custody, federal and military investigators have said.
Cecily Aguilar, a 22-year-old civilian from Killeen, was arrested and charged with one count of conspiring to tamper with evidence for allegedly helping hide Guillen's body.
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The attorney for Guillen's family, Natalie Khawam, said she discovered at least two alleged incidents of sexual harassment -- a superior walking in on Guillen showering and another verbally assaulting her with vulgar remarks in Spanish. Khawam said she's worried about how this case might turn out. She said Guillen told loved ones she didn't want to report the sexual harassment out of fear of retaliation.
But on July 2, Fort Hood officials refuted those allegations, with Special Agent Damon Phelps, who is with the Army's Criminal Investigation Command, saying, "There is no credible information that Specialist Robinson sexually harassed Specialist Guillen."
Guillen's family has been critical of Fort Hood's handling of her case, accusing the Army of lying to them.
In an interview with ABC13's Steve Campion, Guillen's mother called Fort Hood leaders "clowns in a circus."
WATCH: Vanessa Guillen's family speaks out for first time since remains of soldier were identified
Vanessa Guillen's family speaks about her murder
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Over the last couple of weeks, calls have grown for not only a Congressional investigation, but also for Latinas not to join the Army after Guillen's disappearance.
LULAC urges Latinas to avoid joining the military
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