The distinction, between (i) the class of sex offenders who must be released after having completed their prison sentences and (ii) those who could be "civilly" detained, was believed to have been that the later class was made up of individuals who suffered from mental abnormalities which caused them to have "serious difficulty in controlling behavior", thus making them distinguishable "from the dangerous but typical recidivist" that must be released. (Kansas v. Crane (2002) 534 U.S. 407, 413)
