Army investigators believe Guillen, a Texas soldier missing since April, was killed by another soldier on the Texas base where they served, the attorney for the missing soldier's family said Thursday.
Human remains were found Tuesday near the Leon River in Bell County, about 20 miles (32 kilometers) east of Fort Hood, in the search for Guillen.
The criminal complaint released later Thursday said the civilian arrested in connection to the Guillen case is Cecily Aguilar, 22, of Killeen, Texas, near Fort Hood.
From their text conversations, Mayra Guillen said she believed her sister was afraid during her time at Fort Hood.
Guillen was last seen April 22 in a parking lot at Fort Hood.