Deadly force may not be used unless it is necessary to prevent the escape and the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others, according to the case Tennessee v. Garner, 471 U.S. 1 (1985). In addition, when feasible, the suspect must first be warned. State laws and departmental rules can set narrower limits on the use of force, but broader limits are unconstitutional. In another case, Graham v. Connor, 109 S.Ct. 1865 (1989), the Court determined that police officers may be held liable under the Constitution for using excessive force. Such liability must be judged under the fourth amendment's "objective reasonableness" standard, rather than under a "substantive due process" standard. This case gives police officers a "break" in civil liability cases involving the use of force. It recognizes that police officers often make split second judgments in situations that involve their own lives and must, therefore, be judged in the context of a "reasonable officer at the scene."