Heavily armed SWAT teams combed through homes near Boston on Friday in a massive manhunt for one of the Boston Marathon bombing suspects.
But what allows police to search door-to-door for a suspect on the loose without a warrant?
Hours after the FBI released photos and videos of Tamerlan Tsarnaev, 26, and his brother Dzhokhar, 19, clashes between the suspects and police began, the AP reports.
Tamerlan was killed overnight, but his brother remained on the loose Friday afternoon. Officers went door-to-door in several neighborhoods, looking for Dzhokhar.
Generally speaking, the Fourth Amendment protects residents' privacy by typically requiring police to knock and announce their presence before they can enter people's homes, and get a search warrant before they can conduct a search.
But there's an exception for situations in which there isn't time to get a warrant because of an ongoing emergency. When there are exigent circumstances, or emergency situations, police can lawfully enter, search, or seize a resident's property without a warrant.
The exigent circumstance exception exists for the sake of public safety. Often seen on the show "Cops," the classic exigent situation is when the police are in "hot pursuit" of an escaping suspect who is tracked to a private home.
But another example of an exigent circumstance is when further harm or injury could occur in the time it would take to get a warrant. The exception applies to this case, since Dzhokhar is believed to be armed and dangerous, the AP reports. It's entirely possible that he's planning to cause further injury to people.
Officers are also allowed to enter a home without a warrant to help an occupant in an emergency. That means it would be OK for police to enter a house to apprehend Dzhokhar and help a resident who is possibly being held hostage. In such a situation, the police can also do a protective sweep of a house for weapons and other evidence.
One final note about warrantless door-to-door searches: If police do search your home in an emergency, the "plain view" doctrine generally applies. That means officers can seize any contraband they see in, well, plain view -- and that evidence can then be used against you in court.