A Detective received information from a confidential informer, who was also a known narcotic user, that unknown persons were smoking opium in a Hotel. The informer was taken back to the hotel to interview the manager, but he returned at once saying he could smell burning opium in the hallway. The Detective communicated with federal agents and then went back to the hotel with four such agents. All were experienced in narcotics work and recognized at once a strong odor of burning opium, which to them was distinctive and unmistakable. The odor led to a specific room and without knowing who was occupying that room they knocked. A voice inside asked who was there. 'Lieutenant Belland,' was the reply. There was a slight delay, some 'shuffling or noise' and then Johnson (D) opened the door. The officer asked to talk to D. It is alleged by the officers that D stepped back acquiescently admitted the officers. When asked about the opium, D denied that there was such a smell. D was told she was under arrest. The officers searched the room and turned up incriminating opium and smoking apparatus, the latter being warm, apparently from recent use. D was charged. The District Court refused to suppress the evidence and over D's objection admitted it at trial. D was convicted, and the Appeals court affirmed. The Supreme Court granted certiorari.