EquuSearch had spent the last few days looking for Guillen, who is from Houston, near the Leon River. This was the fifth time crews had come out to the location.
The group had called off a search Thursday after founder Tim Miller said they searched everywhere that was "significant." They returned to Killeen Friday after receiving new information from investigators.
It was not clear what new information was learned.
Earlier this week, Congresswoman Sylvia Garcia said foul play is suspected in her disappearance.
The U.S. Representative met with Fort Hood officials Tuesday morning to discuss the search for Guillen.
WATCH: Congresswoman Sylvia Garcia on Vanessa Guillen case
In a statement sent to ABC13 Tuesday night, the U.S. Army confirmed foul play is suspected.
"As we have stated from the day we opened a very extensive investigation on 23 April, we have not ruled anything in or out and are looking at all possibilities," read the statement. "At this point, and after the results of investigative information that has been developed in the last few days, we suspect foul play in the disappearance of PFC Vanessa Guillen."
Fort Hood Deputy Commander Major General Scott Efflandt appeared in a video Sunday evening, pleading for the public's help to find Guillen.
"We want to bring Vanessa home as efficiently and as rapidly as possible. And towards that end, I'm asking for your assistance," Efflandt said. "We need to bring Vanessa back to her Army family and to bring her back to her family, and we won't stop this effort until we're successful."
According to investigators, Guillen's car keys, barracks room key, ID and wallet were found in the armory room where she was working on the base on the day she disappeared.
The attorney for Guillen's family, Natalie Khawam, said she's discovered at least two alleged incidents of sexual harassment -- a superior walking in on Guillen showering and another verbally assaulting Guillen with vulgar remarks in Spanish. Khawam said she's worried about how this case might turn out. She said Vanessa told loved ones she didn't want to report the sexual harassment out of fear of retaliation.
Fort Hood said anyone with information about Guillen is urged to call 254-495-7767. A $50,000 reward is being offered for information as well.
The base said it is already grieving the loss of one soldier, referring to Private Gregory Morales, who had been missing since Aug. 2019.
Morales' remains were found near Fort Hood over the weekend. Officials say foul play is suspected.
Despite social media rumors, authorities do not believe the two cases are related.
Officials are also asking for information in the investigation into Morales' death and said the public can contact Crime Stoppers at 254-526-8477 or submit an online tip via their website.
Foul play suspected in death of Fort Hood soldier Gregory Wedel Morales
Reward increased to $50,000 for missing Fort Hood soldier
Army launches sexual harassment investigation into missing soldier
US Army vows to find missing soldier Vanessa Guillen