Vanessa Guillen, 20, was last seen on April 22 at 1 p.m. in the parking lot of her Regimental Engineer Squadron Headquarters, 3rd Cavalry Regiment.
Her car keys, barracks room key, ID and wallet were found in the armory room where she was working earlier in the day.
But Vanessa's sister says this is unlike her.
"She was a very responsible person. I know she just wouldn't leave her stuff there. I know my sister, she wouldn't leave stuff there if she didn't know she wasn't coming back," Mayra Guillen told KWTX.
Fliers have been posted across Central Texas, and Facebook groups have been created in hopes of bringing Vanessa home safely.
"At this point, I just hope someone comes with a concrete answer. They can tell us, I don't know... something, anything, but something concrete, not just 'We're working on it, we're working on it.' There has to be something," her sister said.
ORIGINAL STORY: Extensive search underway for Fort Hood soldier from Houston missing for nearly a week
Vanessa is described as being of Hispanic descent, 5'2", and 126 pounds with black hair and brown eyes.
Fort Hood Military Police issued a Be on the Lookout notice to surrounding law enforcement, and an extensive search is underway by military members, as well as civilian and military police.
A reward of up to $15,000 is being offered for credible information leading to Vanessa's whereabouts.
Anyone with information is asked to contact Army CID Special Agents at 254-495-7767 or the Military Police Desk at (254) 287-4001.