There's actually a Slate article covering this topic[0]. First, consent was given for certain homes. However, the article also notes that under "exigent circumstances" warrantless searches are permitted:
> In exigent circumstances, or emergency situations, police can conduct warrantless searches to protect public safety. This exception to the Fourth Amendment’s probable cause requirement normally addresses situations of “hot pursuit,” in which an escaping suspect is tracked to a private home. But it might also apply to the events unfolding in Boston if further harm or injury might be supposed to occur in the time it takes to secure a warrant. A bomber believed to be armed and planning more violence would almost certainly meet such prerequisites.
> In exigent circumstances, or emergency situations, police can conduct warrantless searches to protect public safety. This exception to the Fourth Amendment’s probable cause requirement normally addresses situations of “hot pursuit,” in which an escaping suspect is tracked to a private home. But it might also apply to the events unfolding in Boston if further harm or injury might be supposed to occur in the time it takes to secure a warrant. A bomber believed to be armed and planning more violence would almost certainly meet such prerequisites.
[0]: https://slate.com/news-and-politics/2013/04/boston-bomber-ma...