Four drug users bought illegal drugs from Rex Hatfield and his partners (defendants). The buyers, who probably bought illegal drugs from other sources as well, died from ingesting illegal drugs. The United States government (plaintiff) prosecuted the partners for causing the deaths. Strong but inconclusive trial evidence indicated that the partners, rather than other sources, sold the fatal drugs. The trial judge instructed the jury to convict the partners if they found that the partners sold the fatal drugs and that the buyers' deaths "resulted from" their ingestion of those drugs. This part of the judge's instruction was based on the pertinent statutory language. The judge elaborated that the jury should convict the partners if they found that the drugs were "a factor that resulted" in the deaths, and that although the drugs need not have been the "primary cause," they must have "played [at least] a part" in the deaths. The judge overruled the partners' objection to his elaboration. The jury convicted the partners, and they appealed to the United States Court of Appeals for the Seventh Circuit.