Robert Loblaw points to a recent 4th Circuit case, U.S. v. Colonna, 06-5237 (4th Cir., Dec. 20, 2007), suppressing evidence obtained during a search of the defendant's home for child pornography involving 24 FBI agents:

The district court found that Colonna was awakened by armed agents and guarded by agents until the search and interview concluded. The home was inundated with approximately 24 officers who gave Colonna and his family members instructions; that is, they told them where to sit and restricted their access to the home. Colonna did not voluntarily request to speak with Agent Kahn. Instead, Agent Kahn requested that Colonna accompany him to a FBI vehicle to answer questions, wherein a full-fledged interrogation took place. Agent Kahn questioned Colonna for almost three hours, albeit with breaks. But, even during these breaks, Colonna was constantly guarded. Although Colonna was not placed under formal arrest, he was told twice that lying to a federal agent was a federal offense. And, at no time was he given Miranda warnings or informed that he was free to leave.

But the District Court held that since the agents informed Colonna that he was not under arrest, no interrogration took place. Not so, says the 4th Circuit:

Indeed, there is no precedent for the contention that a law enforcement officer simply stating to a suspect that he is "not under arrest" is sufficient to end the inquiry into whether the suspect was "in custody" during an interrogation.

And in a footnote:

Agent Kahn testified that he took twenty-three agents because the house was of considerable size; three stories high, four bedrooms, and a large detached garage.