It's unclear whether Sandy Doan ever suspected the real source of the threatening blackmail messages on her cellphone, but for nearly a year, investigators say, she dutifully obeyed the texts, leaving thousands of dollars at drop-off locations across Dallas.
In return, the person on the other end of the phone said, he wouldn't tell authorities about the math teacher's illicit affair with a student.
Over a year, investigators say, the 27-year-old teacher paid more than $28,000 - unwittingly, to the 14-year-old she was sleeping with.
Dallas Criminal Court documents describe the payments as hush money to cover up an affair that Doan feared would cost her her job and her freedom.
She may lose both anyway. Police arrested Doan on Jan. 20 and charged her with having an improper relationship with a student and with sexual assault. The cycle of sex and blackmail came to an end after a concerned mother tried to figure out where her son was getting so much extra cash, according to an arrest warrant filed in criminal court.
Doan met the student while he attended Quintanilla Middle School, a school on the city's west side where she taught math and social studies and coached the cross-country team, the court documents say. They began communicating via Kik, Instagram and text messages, then started having romantic conversations on the phone.
The relationship got physical in July 2015, according to the affidavit, when Doan picked the student up at his home and drove him to a park to have sex.
A search warrant affidavit obtained by Fox 4 described explicit texts and said Doan sent the student a picture of her naked breasts. They allegedly met for sex in the middle of November and on Dec. 29, 2015.
The threatening texts began a few days later, the affidavit says. Throughout the year, they came from different phone numbers and struck a demanding tone.
"(Ain't) bulls - watch ima start getting the pics and everything ready!" one of the blackmail texts read, according to the affidavit. "To show the cops right I (ain't) playing."
To make the payments, Doan withdrew money from her bank account or took out payday loans.
Even as she was making the payments, she continued having sex with the student, court documents said.
At home, the student, newly enriched, was becoming unruly, his mom told Dallas Fox-affiliate KDFW. He had already been in and out of the juvenile court system for burglary and robbery charges and things were getting worse. He used the money to buy drugs and alcohol. Sometimes he disappeared for days.
"He would probably get money and just leave the whole weekend," she told Fox 4. "I wouldn't hear from him. I'd be out looking for him."
She went through his phone and found the text messages, KDFW reported. She believes the most recent payment was for $1,500 on Jan. 9.
She called the school the next day, trying to get it to put a stop to the relationship. Administrators called police.
Investigators met with Doan and interrogated her about her relationship with the student, and the year of payments, according to the documents. That's when she provided preliminary figures about how much she had paid the student and gave them consent to search her cellphone, the affidavit says. They found evidence of both the blackmail and the sexual encounters.
The Dallas Independent School District and Doan's attorney did not immediately respond to messages from The Washington Post seeking comment.
In 2016, the number of Texas teachers having inappropriate relationships with students climbed for the eighth year in a row, according to the Austin American-Statesman.
The increase, apparently spurred by increasing social media contact, led the Texas Education Agency to ask for nearly $400,000 more to hire investigators to look into the allegations.
"For the past decade, there has been a steady increase in the number of inappropriate educator-student relationships reported to the agency," the agency's budget request states. "As the caseload has increased, the number of investigators has remained the same over the past several years."