(CNN) A Texas sheriff says he has a message for a teenager who's now missing two years after getting probation for a drunken driving crash that killed four people, and for the mother who may be with him.

"You can run, but you're always going to be looking over your shoulder," Tarrant County Sheriff Dee Anderson said Thursday. "We're not going to give up. We're going to come after you. We're going to find you, wherever you are."

The FBI and U.S. marshals have now joined local authorities searching for Ethan Couch , Anderson said.

Couch made national news two years ago when he was sentenced to probation after his lawyers coined an infamous defense strategy, arguing that the teen was a victim of "affluenza" and describing him as a rich kid who was less culpable because his parents didn't set limits for him.

Word of the search for Couch has reignited controversy over the case.

In a statement, Mothers Against Drunk Driving expressed "outrage and disbelief" that the teen has not paid for his crime and called for everyone to be on the lookout as police try to track down Couch.

"Four people were hit and killed, and no one was held responsible," Colleen Sheehey-Church, MADD's national president, said in the statement.

"This is egregious. 'Affluenza' aside, Ethan Couch appears to show blatant disregard for the law, and he must be held accountable. The families impacted will never have their loved ones back; Ethan Couch must have consequences for his actions."

Lawyers: Mother is also missing

Couch's probation officer couldn't reach him this week, his lawyers told CNN. Couch's mother, with whom the 18-year-old had been living, is also missing, said attorneys Scott Brown and William Reagan.

Under the terms of his probation, Couch isn't supposed to lose contact with his probation officer, his lawyers said.

Their apparent disappearance comes shortly after the video of Couch at a party turned up on social media, CNN affiliate KTVT reported.

When asked to confirm Couch's attendance at the event, his attorneys told CNN's sister network HLN they "are aware that the probation authorities and/or the District Attorney's office are conducting an investigation to determine if Ethan has violated any of the terms and conditions of his probation. It would not be prudent for us to make any further statement on Ethan's behalf until the investigations are concluded and it is determined what, if any, action will be taken against him."

Anderson told CNN's "AC360" he's worried Couch and his mother could have fled the United States.

"They have the money. They have the ability to disappear, and I'm fearing that they have gone a long way and may even be out of the country," he said.

Jail time possible if probation terms violated

Couch's attorneys argued in 2013 that his parents, because they spoiled him, were partly to blame for the crash on a road in Burleson, south of Fort Worth.

Prosecutors had asked for 20 years behind bars, but a Tarrant County juvenile court judge sentenced Couch to a decade of probation. Couch was ordered into long-term mental health treatment, away from his parents' influence.

After his recent disappearance, a court ordered Couch detained, according to his lawyers. At the time of his conviction, prosecutors said Couch could face up to 10 years of incarceration if he violated the terms of his probation.

The approach was met with widespread outrage and disdain, and the term "affluenza" was widely mocked.

Drunken crash

On the night of June 15, 2013, Couch and some friends stole beer from a Walmart.

That same evening Hollie Boyles and her daughter Shelby left their home to help Breanna Mitchell, whose SUV had broken down by the side of a road. Brian Jennings, a youth pastor, was driving past and also stopped to help.

Couch plowed into them, killing them all. The crash threw two passengers out of the bed of Couch's truck, injuring both severely.

The parents of one of the injured teens, who suffered debilitating brain injuries, sued Couch's family for $2 million.

Three hours after the crash, tests showed that Couch had a blood-alcohol content of 0.24, three times the legal limit. Couch's vehicle also struck a parked car, which then slid into another vehicle headed in the opposite direction.

Eric Boyles, Hollie's husband and Shelby's father, felt the judge had been far too easy on Couch and angrily spoke out against how Couch was treated. The message the judge sent is all wrong, he said.

"The primary message has to absolutely be that money and privilege can't buy justice in this country," he said.