Dunn was last seen on December 27, 2010. The teenager was 13 years old at the time.
On December 27, 2012, volunteers with Hope For Hailey and others used flower petals to retrace the steps they believed Dunn took in Colorado City before she disappeared.
Although there have been several flurries of activity in the 2 years following Dunn's disappearance -- including the search of a Big Spring field and recent revelations from a federal prisoner indicating that Dunn might have been held captive in the Odessa area -- no sign of Dunn has been found.
In March, human remains were found near the Big Spring Airpark. The remains were later determined to be that of a man.
In October, letters written by federal prisoner Sonja Renee Callahan were released. The letters indicated that a young girl fitting the description of Dunn was held captive by drug traffickers near Odessa.
The letter also indicates that Dunn was killed and buried.
However, law enforcement hasn't commented further on the investigation into the contents of the letter.
The only named person of interest in the disappearance is Billie Dunn's former boyfriend Shawn Adkins.
Adkins has not been charged with a crime or even listed as a suspect.