(COLUMBIA, Tenn.) -- It's been three weeks since 50-year-old Tad Cummins and 15-year-old Elizabeth Thomas disappeared on March 13, but investigators say they hope surveillance footage of the pair at Walmart will help crack the case.
The only confirmed sighting of the pair has been at that Walmart in Oklahoma City, Oklahoma, two days after they disappeared. The footage was released by authorities Friday.
The TBI said that Cummins, who was fired one day after the alleged kidnapping, "may have been abusing his role as a teacher to groom [the teen] ... in an effort to lure and potentially sexually exploit her."
Cummins is wanted on allegations of aggravated kidnapping and sexual contact with a minor and an Amber Alert has been issued for Elizabeth.
The March 15 Walmart sighting in Oklahoma City was nearly 700 miles away from the teen's home in Columbia, Tennessee. The photos showed small changes to their appearances -- Elizabeth's hair was darkened red and Cummins' gray facial hair was turned brown.
Former FBI Agent and ABC News contributor Brad Garrett told ABC News Monday he considers the Oklahoma sighting the best lead in the case, even though it actually happened nearly three weeks ago.
"I think this photo and this location are terrific because it tells the public, at least in mid-March, that they weren't really going out of their way to conceal who they are," Garrett said, citing how the duo didn't alter their appearances in a major way, didn't wear disguises and went to a store that has security cameras.
"If you're going be bold enough to walk into some place like Walmart ... you're going get caught on camera," Garrett said. "Tad didn't think through that, [which] goes to the unsophistication of his ability to be a fugitive."
But, Garrett added, Cummins has figured out a way to elude law enforcement, which he thinks is likely by using a vehicle that authorities doesn't know about.
The TBI said last week that "efforts to determine what vehicle they were traveling in [at Walmart] remain ongoing."
Charles Crowson, senior manager of corporate communications at Walmart, said in a statement last week, “We are aware of this situation and will continue helping law enforcement with their investigation."
The TBI has received 1,328 tips in this case, 302 of which remain open and are being pursued, TBI spokeswoman Susan Niland told ABC News Monday.
She called the Oklahoma sighting "helpful to us since it's the only sighting we have been able to confirm thus far."
Josh DeVine, a TBI spokesman, said Friday, "We are encouraged by the sighting. ... They could still be anywhere, but [the sighting] proves that we need the national public to stay vigilant. We must encourage the public to keep their eyes open."
While the case has now reached the three-week mark, Garrett thinks the duo will "eventually be found."
Garrett said the Oklahoma sighting "gives me hope that there's going to be other sightings of them at these vulnerable junctures for fugitives -- which are food, shelter and transportation."
Going forward, Garrett thinks law enforcement should focus on Cummins' friends, as well as places Cummins has a connection to.
Law enforcement would likely "want to keep a very tight circle around Tad's friends and associates, both in middle Tennessee and around the country," Garrett said.
Cummins will get desperate for money eventually, Garrett explained, since it would be too difficult for him to go to an ATM, he will likely reach out to his friends for help, so it's imperative that law enforcement is aware if he reaches out to those friends.
Also, Garrett said, "I think anytime people are on the run, they still have to go in a direction where they think they can seek food, shelter and safety. And many times that means going to places where you have a history."
For Cummins, that could be, for example, a location connected to "a former job, an ex-wife, children, former friends or current friends."
As the search continues, Elizabeth's family is praying and pleading for the 15-year-old's safe return.
At a candlelight vigil in her hometown of Columbia, Tennessee, Sunday night, Elizabeth’s father said to the crowd of more than 100 people, "Without her, the house is just too quiet."
"Please understand that she is not better off out there," he said, according to ABC affiliate WKRN-TV. "She is better off here."
Authorities are asking that anyone with information call 1-800-TBI-FIND and that anyone who sees a car with Tennessee license plate number 976-ZPT call 911.
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