Tarrant County Sheriff Dee Anderson told INSIDE EDITION: "My biggest fear is that they have gone somewhere where we are not going to find them at all."
Authorities have been unable to contact 18-year-old Ethan and his mother Tonya since he missed an appointment with his probation officer near Fort Worth, Texas, last week.
The sheriff says Ethan Couch is top of his most-wanted list and that he knew the teen was trouble.
“I said then that we should have him in jail, he should not be out there walking the streets. I hate to say I told you so but I told you so,” the sheriff said.
Many are also asking what role the mom is playing in the disappearance.
An article in the Dallas magazine, D, earlier this year painted a bizarre picture of the mother-son relationship.
The founder of a school Ethan attended said of Tonya: "She loved the boy so much that she couldn't say no to him."
The psychiatrist who came up with the phrase "Affluenza" at Ethan’s DUI sentencing in 2013 said Tonya was "addicted to the kid."
"Instead of the golden rule, Ethan was taught 'we have the gold, we make the rules,” the expert said at the time.
A video deposition obtained by 20/20 of Tonya Couch revealed the mother couldn't remember the last time she disciplined her son.
"I don't remember," the mother said when she was deposed last year in a lawsuit over the DUI crash, in which her son killed four people.
She divorced Ethan’s father Fred, a sheet-metal millionaire, in 2006.
Inside the far-from-impressive rented house where Ethan and his mom lived for a year right before they disappeared has a modest kitchen and the bathroom looks more like it's in a motel.
The house is about four hundred miles from the Mexican border.
The teen’s disappearance comes after video surfaced of him yucking it up at a beer-pong party - even though he was under court order not to use alcohol.
Prosecutors were investigating whether he was drinking at the party which would be a violation of his 10-year-long probation.