Sonya Wallace lived near the post office. It was just four blocks from her family’s home in Rockdale, Texas.

So when Sonya, 15, left home on the afternoon of February 19, 1999, to go mail a letter, her family expected her back shortly.

Sonya never returned.

Sonya Wallace Williamson County Sheriff's Office

“Her family noticed right away that she didn’t come back,” Detective Rex Miller, now retired from the Williamson County Sheriff’s Office, told Dateline.

Sonya’s mother Linda Gonzalez told Dateline she wasn’t home when Sonya left the house.

“I will regret this till the day I die – that was supposed to be my day off. But they called me in to work, and so I went to work” Linda told Dateline. “I didn’t find out until I got back that night at 10:00 p.m.”

Sonya’s family reported her missing the following day. The Rockdale Police Department had originally been assigned to work the case, as that is where Sonya had last been seen. Authorities say investigators there interviewed dozens of people at the time, including Sonya’s friends, family, and her fellow students at Rockdale High School.

Weeks went by with no sign of Sonya or leads in her case. But on March 14, about a month after Sonya disappeared, a local rancher made a gruesome discovery about 40 miles from Sonya’s home.

“A rancher had cattle in the area and he saw activity below the bridge – vultures. He stopped to see what they were circling,” Det. Miller said. They were circling what would later be identified as Sonya’s remains. Detective Miller said Sonya had been bludgeoned to death, and the remains were in a state of severe decomposition.

“It was obvious that foul play was involved,” Det. Miller told Dateline. “We have no evidence of where the actual murder occurred. My belief is that she was killed elsewhere and brought under the bridge.”

Sonya Wallace Williamson County Sheriff's Office

Sergeant John Pokorny of the Williamson County Sheriff’s Office told Dateline that his office then took over the case, since Sonya’s remains had been found within their jurisdiction. He said evidence was gathered from the scene – Sonya’s clothing, soda bottles – and the office’s homicide team began an active investigation.

“I couldn’t function. I wouldn’t come out of the bedroom. My sister would drag me out and say I had to face the people who came to pay their respects,” Sonya’s mother Linda said. “Even after they found her, I would still see people in stores who looked like her. So I threw myself into my work. It still is hard, even to this day.”

For nearly 18 years, few tips would come in as to what happened to Sonya Wallace. But soon after he was elected to the Williamson County Sheriff’s Office in 2016, Sheriff Robert Chody formed a Cold Case Unit which was assigned to take a fresh look at the teen’s case.

Linda said authorities came to her house when the Cold Case Unit was formed to tell her the news.

“They came to the house to see me and I was so excited,” she said. “They are awesome. I honestly believe that they’re the ones who are going to solve this.”

Sgt. Pokorny said that while agencies across the country have different guidelines for what is considered a cold case, “For us, it’s when a detective runs out of leads and it’s at a standstill. The cases were investigated and worked, but then put in a corner.”

Sonya’s case is now one of 13 cold cases the Williamson County Sheriff’s Office is reexamining.

“With any cold case, we are able to read the first page of the original report,” Sgt. Pokorny told Dateline. “We kind of act like it had just happened. We go back to the crime scene. Policing has changed a lot since 1999, so now we can identify things that should have been looked at back then.”

Twenty years after Sonya’s disappearance, Sgt. Pokorny and his unit say they have identified 300 people who could know something about what happened to the teenager.

“She was in high school. She was very sociable,” Det. Miller said. “We’re going to talk to people one at a time, and see what we find. It’s painstaking, but we have to do it.”

Authorities say they’ve already spoken to about 30 of those people since re-opening Sonya’s case. Linda told Dateline she believes her daughter’s killer is someone Sonya knew. But even if someone is convicted in her daughter’s case, Linda says, “It’s not going to be relief. I don’t believe in the death penalty. So I really don’t know what kind of closure it would bring. There’s no closure. There’s never going to be any closure.”

DNA recovery technology is also more advanced now than when Sonya’s remains were first found, and the unit plans to re-scan Sonya’s clothing and fingernails.

“In 1999, they used Q-Tips to swab her clothes,” Sgt. Pokorny said. “[The new technology] goes into fibers of shirts to get DNA back much better than Q-Tips do.”

Det. Miller, though now retired, still works in an official capacity on Sonya’s case and says he speaks to Sonya’s family on a regular basis.

“When a case lays dormant, the families get discouraged,” Det. Miller said. He wants Linda and the rest of Sonya’s family to know that even 20 years later, he and his fellow cold-case investigators are not giving up on Sonya’s case.

“No matter how small or minute it may be, [people] need to report what they know because it could lead to something,” Sgt. Pokorny said.

If you have any information on the circumstances surrounding Sonya Wallace’s disappearance and murder, please call the Williamson County Sheriff’s Office Cold Case Unit at 512-943-5204 or email coldcasetips@wilco.org. Tips may remain anonymous.