Erroneous 'Instant Alibi' Hampered Investigation Into Fort Hood Soldier Vanessa Guillén's Disappearance, Army Officials Say

New details are being released about the killing of Fort Hood soldier Vanessa Guillén, including how an erroneous alibi initially delayed the investigation.

Guillén disappeared from the military base on April 22, sparking an extensive search for the 20-year-old Houston native. Her dismembered remains were later discovered near the Leon River in Belton, Texas.

Investigators believe Guillén was killed by U.S. Army Specialist Aaron Robinson, who had been the last person to see Guillén alive, but Maj. Gen. Donna Martin, the Army’s provost marshal, said in a new interview on Friday’s “20/20” that investigators were initially thrown off Robinson’s track because of an erroneous alibi, according to ABC News.

Guillén worked maintaining weapons on the base would have come in contact with Robinson, who was also stationed in a separate arms room. Three soldiers told authorities that on the night Guillén disappeared, they had seen her at “a time that would’ve indicated she had left Spc. Robinson’s arms room” before she disappeared.

“That led us to the parking lot, where they said they saw her walk,” Martin said. “We did a lot of investigation out into parking lot to see, maybe if she had been abducted [there].”

Investigators used search dogs but “didn’t find anything,” she said. The investigation was further hampered because that area of the parking lot did not have cameras to confirm the sighting.

Ryan McCarthy, secretary of the U.S. Army, said the soldiers submitted affidavits saying they had seen Guillén at a different time from when she had actually left the arms room where she worked to go to the separate arms room where Robinson was working. The inaccurate witness accounts essentially gave Robinson an “instant alibi,” McCarthy said.

“The trail went cold for about a month,” he said.

But attention turned to Robinson once again after investigators discovered that he had called his girlfriend Cecily Aguilar multiple times on the night Guillén vanished, despite telling authorities that he had gone to the home he shared with Aguilar after work and that the two had spent the rest of the night together, according to a criminal complaint in the case obtained by KDH News.

Several witnesses also reported seeing Robinson leave work that day with a “tough box” with wheels that appeared to be very heavy. He loaded the box into his vehicle and drove away.

Aguilar initially told investigators that Robinson had called her because she had lost her phone and asked him to call it to help her find it, but she later allegedly admitted that Robinson had called her around 8:30 p.m. on the night Guillén disappeared to tell her that he had killed a woman by hitting her in the head with a hammer multiple times at the arms room where he worked, the criminal complaint said.

Aguilar told authorities Guillén had “never made it out of the Army alive” and allegedly admitted to helping Robinson dismember and bury Guillén’s body near the river, according to the court documents.

Authorities discovered the remains on June 30.

After the discovery, Martin said Robinson was held in a room being watched by an unarmed escort; however, he had not been officially detained at the time.

Investigators believe he fled the base after seeing local media reports on his phone about Guillén's body being found.

“He gets in a vehicle and he flees and he leaves Fort Hood,” Martin said, adding that local law enforcement immediately gave chase and pulled Robinson over.

He shot himself to death as police closed in, authorities said.

Martin declined to say how he got the firearm but told “20/20” it was “not a government weapon” and had not gotten it from the arms room where he worked.

Guillén’s mother, Gloria Guillén, has said that her daughter was being sexually harassed by someone on the Army base at the time of her death and believes that Robinson may have been the culprit.

“My daughter didn’t give me a name,” she told “20/20.” “But I begged Vanessa’s friend so much. … And she said, ‘Yes, yes, there is a man: Robinson.’”

The family’s attorney Natalie Khawam said that someone had once followed Guillén into a shower and another person had harassed her with “vulgar words.”

But Martin told the news outlet that Robinson had not been involved in any sexual harassment before Guillén's death.

“In our criminal investigation, we found no evidence of sexual harassment between Vanessa and Spc. Robinson. … That was ruled out very early,” she said.

She also said that the shower incident may have been a misunderstanding with another soldier who accidentally approached Guillén while she was using a baby wipe to clean herself during a field exercise, in a practice known as a “hygiene shower.”

“She was actually behind a bush and she was conducting field sanitation,” Martin said. “Her platoon sergeant may have walked by and heard some noise. … He called out and said, ‘Is there someone there?’ She identified herself. And he said, ‘What are you doing?’ And she says, ‘I'm conducting personal hygiene.’ And that was the encounter.”

Aguilar was indicted in July by a federal grand jury on one count of conspiracy to tamper with evidence and two substantive counts of tampering with evidence, according to a press release from The United States Attorney’s Office Western District of Texas.

Authorities have alleged that she “conspired with Robinson to corruptly alter, destroy, mutilate and conceal evidence, including the victim’s body in order to prevent Robinson from being charged with and prosecuted for any crime.”

Her close friend Ayrren Clough told “20/20” that she still struggles with the horrific allegations against her friend.

“I said, ‘There’s no way,’” Clough said. “I just can’t see my best friend doing that to another human. You know, she has a heart. She’s kind to people. And she has a very weak stomach, especially. And for them to say what she was doing and had done, I just couldn’t believe it.”

Aguilar was the estranged wife of another former Fort Hood soldier, Keon Aguilar, at the time of the slaying. Clough said Aguilar began dating Robinson some time after he had moved in with the couple in December 2019.

“She had told me that when she moved out of Keon’s house that Aaron had came with her … and they were dating,” she said, adding that she believed the relationship began because Aguilar was having trouble in her marriage at the time.

She believes Aguilar, who had been in the foster care system as a teen, may have agreed to help Robinson out of fear.

“The only way I can see her doing something like this is for the fear of her life,” she said.

Aguilar has pleaded not guilty to the charges against her.