DOHA, Qatar — Matthew and Grace Huang, an American couple accused of killing their 8-year-old adopted African daughter last year by depriving her of sustenance for four days, were unexpectedly found guilty on Thursday and were each sentenced to three years in prison followed by deportation, in a case that has drawn close attention in Qatar and in the United States.

The State Department expressed disappointment at the verdict, which the defendants and their supporters described as a grievous error that would be appealed.

The Huangs were allowed to leave the courtroom after the verdict and remained free on their own recognizance, as they have been for the past four months after nearly a year of incarceration in Qatar, where Mr. Huang had been working as a public works infrastructure engineer. Their legal representatives said that the appeal process would most likely begin in May and that they would seek permission to fly home even while the appeal is pending.

“We have just been wrongfully convicted, and we feel as if we are being kidnapped by the Qatar judicial system,” Mr. Huang, 37, said in a statement that he read to reporters outside the courtroom with his wife, 36, standing beside him. “This verdict is wrong and appears to be nothing more than an effort to save face.”