Ex-teacher who allegedly kidnapped teen planned to run to Mexico, prosecutors say Tad Cummins was on the run for over a month after allegedly kidnapping the teen.

 -- The former Tennessee teacher who authorities say kidnapped his 15-year-old student then allegedly spent over a month on the run with her had planned to flee to Mexico, federal prosecutors said.

Tad Cummins, 50, a married father and grandfather, went missing with 15-year-old Elizabeth Thomas on March 13, authorities said. An Amber Alert was issued for Elizabeth, while Cummins was wanted on allegations of aggravated kidnapping and sexual contact with a minor. The duo was found on April 20; the teen was "healthy and unharmed," authorities said, and Cummins was taken into custody.

Cummins allegedly plotted their getaway from the moment he was suspected of having an improper relationship with the teen, according to a motion filed by federal prosecutors today supporting detention for Cummins.

"The logical inference is that the defendant ... fled to avoid criminal charges," prosecutors said.

Prosecutors said Cummins planned to reach the Mexico border and then head to countries further south.

Cummins allegedly obtained a "small watercraft and conducted a test run to cross into Mexico across the water from San Diego," prosecutors said. "The defendant also considered the feasibility of a land crossing into Mexico."

Prosecutors claim that in an effort to evade capture, Cummins deliberately left a "misleading note with his wife regarding" which direction he was traveling. Cummins also allegedly altered his appearance, switched license plates twice, disabled the car's GPS system and used aliases for himself and the teenager, prosecutors said.

Cummins and Elizabeth were found on April 20 at a remote cabin in northern California near the Oregon border after over a month on the run.

Cummins was arrested on a state warrant for aggravated kidnapping and he faces a federal charge of transporting a minor in interstate commerce with the intent to engage in criminal sexual activity. He appeared in federal court in Sacramento, California, today, where he did not enter a plea. Cummins is expected to be transferred to Tennessee.

The teenager was found "healthy and unharmed," authorities said. She has returned to Tennessee and is in a "safe location with family and friends where she is comfortable and resting," said attorney Jason Whatley, who is representing the Thomas family.

Cummins, who was fired one day after the alleged kidnapping, had allegedly researched teen marriage online, specifically the age of consent, according to law enforcement officials.

One of Elizabeth's schoolmates reported seeing her and Cummins kiss in his classroom on Jan. 23, according to a school district investigative report, but both denied the claim. A school report from January reads that neither one "admitted to behaving inappropriately towards the other."

Elizabeth’s father, Anthony Thomas, told ABC News after Elizabeth was found, “She may not be exactly ... the person she was because there’s a lot of experiences she’s had."

"I'm not allowed to ask her about things that happened along the way right now," he said.

Elizabeth’s father said the family is instead keeping "things positive."

"I go in there and tell her how much I missed her, how much I love her and how much her dog missed her," Anthony Thomas said.

"I think she has the determination to really go somewhere in life," he said. "But right now she really needs a lot of help."

ABC News' Annie Pong contributed to this report.