FORT WORTH, Texas -- Federal officials have joined Texas deputies in their search for Ethan Couch, a teenager made famous when he told a court he suffered from "affluenza," a condition resulting from being coddled into a sense of irresponsibility.

Couch, who was 16 years old when he killed four pedestrians in a drunk driving crash, was sentenced in 2013 to 10 years of probation. He has allegedly been out of contact with authorities for several days, reports CBS DFW. His mother is missing as well.

Tarrant County Sheriff Dee Anderson told CBS DFW that he would not be surprised if Couch is no longer in the country. Anderson said Couch is now his office's top fugitive and on Wednesday issued a warrant for the 18-year-old's arrest. Thursday announced that U.S. Marshalls and the FBI have joined the search.

Just learned US Marshall & FBI are formally joining the search for Ethan Couch. We welcome their help. #EthanCouch pic.twitter.com/KU4XtiF9n5 — Dee Anderson (@SheriffAnderson) December 17, 2015

Couch's sentence received national attention after a defense witness used the term "affluenza" to describe his family's wealth contributing to his lack of responsibility.

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It was a decision the sheriff criticized.

"I hate to say I told you so, but I told you so," Anderson said. "I said, we're going to see him again. He's going to be back in this system."

Anderson was not aware of any restrictions on travel for Couch. No arrest warrant was issued for Couch's mother Tonya, but Anderson said they were looking for anyone who may be with him or helped him leave the area.

Anderson said there were indications Couch had left his last known residence for some time. He believes the disappearance was planned after the release of a video earlier this month, showing a boy who appeared to be Couch at a party playing a drinking game.

That video would not necessarily be a probation violation, according to Dallas attorney Nicole Knox.

"The state would have to prove that this was him in the video and it was taken at a time and occurred at a time that was during his probation instead of before it," she said.

However, not being in contact with probation was the easiest requirement to fulfill she said, and is an immediate problem for Couch's future.