In Costello v. United States, the U.S Supreme Court ruled in favor of Frank Costello, a reputed mobster who was facing deportation from the United States due to his criminal activities. The government had sought to deport him based on evidence that he had committed perjury during his naturalization proceedings. However, the court held that under Section 340(a) of the Immigration and Nationality Act (INA), denaturalization could only occur if it was proven that citizenship was procured illegally or through misrepresentation. In this case, while there were allegations of perjury against Costello during his naturalization process, no concrete proof existed showing he obtained citizenship fraudulently or unlawfully. Therefore, despite acknowledging Costello's notorious reputation as a gangster and admitting that he may have lied about not being involved in illegal activities at the time of gaining citizenship; without clear evidence proving fraudulent procurement of nationality status - which is what section 340(a) requires for denaturalization - they couldn't strip him off his American Citizenship.

In the dissenting opinion for Costello v. United States, Justice Frankfurter expressed his disagreement with the majority's decision to overturn Costello's deportation order. He argued that while it was true that the government had relied on hearsay evidence in its case against Costello, this did not necessarily mean that due process had been violated. According to him, hearsay can be used as evidence in administrative proceedings such as immigration hearings if it is deemed reliable and trustworthy by a competent authority. Furthermore, he pointed out that there were other pieces of non-hearsay evidence presented at trial which supported the conclusion reached by both lower courts - namely, that Costello should be deported because he was involved in organized crime activities during his time as a U.S citizen.