The Fourth Amendment “right of the people to be
secure in their persons . . . against
unreasonable searches and seizures” generally requires a law enforcement officer to have probable cause for conducting a search. “Probable cause exists where ‘the facts and cir stances within [an officer’s] knowledge and of which [he] had reasonably
trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that’ an offense has been or is being committed,” Brinegar v. United States, 338 U. S. 160, 175–176 (1949) (quoting Carroll v. United States, 267 U. S. 132, 162 (1925)), and that evidence bearing on that offense will be found in the place to be searched.
In T. L. O., we recognized that the school setting “re-quires some modification of the level of su ion of illicit activity needed to justify a search,” 469 U. S., at 340, and held that for searches by school officials “a careful balancing of governmental and private interests suggests that the public interest is best served by a Fourth Amendment standard of reasonableness that stops short of probable cause,” id., at 341. We have thus applied a standard of reasonable su ion to determine the legality of a school administrator’s search of a student, id., at 342, 345, and have held that a school search “will be permissible in its scope when the measures adopted are reasonably related to the objectives of the search and not excessively intrusive in light of the age and sex of the student and the nature of the infraction,” id., at 342.
A number of our cases on probable cause have an implicit bearing on the reliable knowledge element of reasonable su ion, as we have attempted to flesh out the knowledge component by looking to the degree to which known facts imply prohibited conduct, see, e.g., Adams v. Williams, 407 U. S. 143, 148 (1972); id., at 160, n. 9 (Mar-shall, J., dissenting), the specificity of the information received, see, e.g., Spinelli v. United States, 393 U. S. 410, 416–417 (1969), and the reliability of its source, see, e.g., Aguilar v. Texas, 378 U. S. 108, 114 (1964). At the end of the day, however, we have realized that these factors cannot rigidly control, Illinois v. Gates, 462 U. S. 213, 230 (1983), and we have come back to saying that the standards are “fluid concepts that take their substantive con-tent from the particular contexts” in which they are being assessed. Ornelas v. United States, 517 U. S. 690, 696 (1996).
Perhaps the best that can be said generally about the required knowledge component of probable cause for a law enforcement officer’s evidence search is that it raise a “probability,” Gates, 462 U. S., at 238, or a “substantial chance,” id., at 244, n. 13, of discovering evidence of criminal activity. The lesser standard for school searches could as readily be described as a moderate chance of finding evidence of wrongdoing.