FORT WORTH, Texas – A Texas woman and her son, a teen known for invoking an “affluenza” defense after a deadly drunken driving wreck, held a sort of farewell party before fleeing his probation, driving to the Mexican border and trying to disguise themselves, officials said Tuesday.

Tarrant County Sheriff Dee Anderson said 18-year-old Ethan Couch and his mother, who were detained Monday in the Pacific Coast resort city of Puerto Vallarta, planned their disappearance.

“They even had something that was almost akin to a going-away party before leaving town,” he said.

Couch was on juvenile probation for the wreck that killed four people when he was 16. During the sentencing phase of his trial, a defense expert argued that his wealthy parents coddled him into a sense of irresponsibility – a condition the expert termed “affluenza.” The condition is not recognized as a medical diagnosis by the American Psychiatric Association, and its invocation drew widespread ridicule.

Anderson said an arrest warrant would be issued for Couch’s mother, Tonya Couch, on charges of hindering an apprehension. A prosecutor says that during a hearing next month they plan to ask a judge to transfer Ethan Couch’s case to adult court.

Couch’s attorneys, Scott Brown and Reagan Wynn, said in a statement they won’t comment on the case until they speak with their client, which likely won’t happen before Couch reaches the U.S.

Ricardo Ariel Vera, the representative of Mexico’s immigration institute in the western state of Jalisco, said the mother and son were being held at immigration offices in the state capital, Guadalajara, and would be returned to the United States aboard a commercial flight to Houston sometime Tuesday.

“They are going to be sent back to their country, given that they were in Mexico improperly,” Ariel Vera said. “They would have had to enter, for example, as tourists, but they entered without registering.”

Mexico’s Jalisco state prosecutors’ office said its agents had been working with American authorities since Dec. 26 to track down and capture Couch and his mother.

They were found in a dowdy section of Puerto Vallarta’s old town, far from the glitzy resorts, golf courses and high-rise hotels of the city’s newer section. The street corner where they were found is dotted with a small sandwich shop, a taco stand, and a mom-and-pop corner store. A playground and a day-care center with a fence topped with razor wire stand nearby.

Couch was apparently trying to lie low; a photo distributed by the Jalisco state prosecutor’s office show him in detention with his blond hair dyed black and his normally blondish beard a light brown.

Anderson has said he believes the two fled in late November after a video surfaced that appears to show Couch at a party where people were drinking. If found to be drinking, Couch’s probation could be revoked and he could be sentenced to up to 10 years in prison.

Couch was driving drunk and speeding on a dark two-lane road south of Fort Worth in June 2013 when he crashed into a disabled SUV off to the side, killing four people and injuring several others, including passengers in Couch’s pickup truck.

He pleaded guilty to four counts of intoxication manslaughter and two counts of intoxication assault causing serious bodily injury. Because of his age, he wasn’t certified as an adult for trial and a judge sentenced him in juvenile court to 10 years’ probation and a stint in a rehabilitation center.

Authorities had begun searching for the pair after Ethan Couch failed to keep a mandatory appointment with his probation officer on Dec. 10, leading authorities to issue the juvenile equivalent of an arrest warrant for him.

The U.S. Marshals Service had issued a wanted poster promising a reward of up to $5,000 for information leading to Ethan Couch’s whereabouts and capture.