Mexican authorities have issued alerts for the 32-year-old Queenslander and her Mexican husband Jorge Luis Aguirre Astudillo, also 32, who haven't been seen May 2.
The alarm was raised after the couple's toddler Adelynn was found wandering the streets alone near a church in the resort city of Cancun in the country's eastern Quintana Roo state.
Ms Shanks' family have taken the young girl back to Queensland, while authorities probe her parents' disappearance.
Quintana Roo Attorney-General Oscar Montes de Oca Rosales says the couple have either been targetted by drug cartels, or it's linked to domestic violence.
"It could be just retaliation from the criminal groups that he was involved with," he told Nine's 60 Minutes program on Sunday night.
Mr Montes de Oca Rosales said it was possible Mr Astudillo could be a cartel member.
The couple and their daughter were last seen driving on a toll road out of Cancun about 11.30am on the day of their disappearance.
However, their car was not clocked returning through the same checkpoint later that day.
"What I can say is well maybe they were being followed and that's why they turned around and took the secondary road," Mr Montes de Oca Rosales said.
"He (Jorge) had been threatened before and he even had hired a bodyguard … and he was afraid for his life."
Ms Shanks' brother Dan said Mr Astudillo was seen dropping off the couple's daughter near a church and fleeing the scene.
"That's the thing, she's just disappeared, both of them are gone, and then a few hours later Ade's (Adelynn) at a church in Cancun," he told the Nine program.
The attorney-general said there' was not enough evidence to determine if either Ms Shanks or Mr Astudillo were alive or not.
"According to the law we consider them as alive, and that's the way that we look for them until something is shown to be different," Mr Montes de Oca Rosales said.
Quintana Roo authorities have been analysing footage of a car leaving the couple's home, and other CCTV footage and pinging the couple's mobile phones to trace their last GPS locations.
Ms Shanks' mother Leanne is looking after her granddaughter Adelynn, and says the family is extremely worried about Tahnee.
"That many things (are) going through my head, you know, like I don't want to her to be suffering anywhere, you know," she sobbed.
"But look, it's just about ... we've got to do everything we can to try and find her, we are distraught, her brothers and sister love her ... we've just got to bring her home to Ade."
Ms Shanks and Adelynn had been planning to fly back to Queensland on June 16, her family says.
According to her Facebook profile, Ms Shanks, from Conway Beach in the Whitsundays, had been trying to get home after her plans were apparently delayed by the coronavirus pandemic.
She wrote in May 2021: "I can't come back to Australia till mid 2022!!! My baby girl will be 2 1/2 years old and hasn't even met her grandad, uncles, great gran, cousins."
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