UNITED STATES V. BRIGNONI-PONCE, 422 U. S. 873 (1975)
Argued February 18, 1975

Decided June 30, 1975

422 U.S. 873

parts of decision
The Government makes a convincing demonstration that the public interest demands effective measures to prevent the illegal entry of aliens at the Mexican border. Estimates of the number of illegal immigrants in the United States vary widely. A conservative estimate in 1972 produced a figure of about one million, but the INS now suggests there may be as many as 10 or 12 million aliens illegally in the country. [Footnote 4] Whatever the number, these aliens create significant economic and social problems, competing with citizens and legal resident

Page 422 U. S. 879

aliens for jobs, and generating extra demand for social services. The aliens themselves are vulnerable to exploitation because they cannot complain of substandard working conditions without risking deportation. See generally Hearings on Illegal Aliens before Subcommittee No. 1 of the House Committee on the Judiciary, 92d Cong., 1st and 2d Sess., ser. 13, pts. 1-5 (1971-1972).

The Government has estimated that 85% of the aliens illegally in the country are from Mexico. United States v. Baca, 368 F.Supp. 398, 402 (SD Cal 1973). [Footnote 5] The Mexican border is almost 2,000 miles long, and even a vastly reinforced Border Patrol would find it impossible to prevent illegal border crossings. Many aliens cross the Mexican border on foot, miles away from patrolled areas, and then purchase transportation from the border area to inland cities, where they find jobs and elude the immigration authorities. Others gain entry on valid temporary border-crossing permits, but then violate the conditions of their entry. Most of these aliens leave the border area in private vehicles, often assisted by professional “alien smugglers.” The Border Patrol’s traffic-checking operations are designed to prevent this inland movement. They succeed in apprehending some illegal entrants and smugglers, and they deter the movement of others by threatening apprehension and increasing the cost of illegal transportation.

Against this valid public interest we must weigh the interference with individual liberty that results when an officer stops an automobile and questions its occupants.

It continues further on down the ruling;

“The Fourth Amendment does not require a policeman who lacks the precise level of information necessary for probable cause to arrest to simply shrug his shoulders and allow a crime to occur or a criminal to escape. On the contrary, Terry recognizes that it may be the essence of good police work to adopt an intermediate response. . . . A brief stop of a suspicious individual, in order to determine his identity or to maintain the status quo momentarily while obtaining more information, may be most reasonable in light of the facts known to the officer at the time.”

Id. at 407 U. S. 145-146.

These cases together establish that, in appropriate circumstances, the Fourth Amendment allows a properly limited “search” or “seizure” on facts that do not constitute probable cause to arrest or to search for contraband or evidence of crime. In both Terry and Adams v. Williams, the investigating officers had reasonable grounds to believe that the suspects were armed and that they might be dangerous. The limited searches and seizures in those cases were a valid method of protecting the public and preventing crime. In this case as well, because of the importance of the governmental interest at stake, the minimal intrusion of a brief stop, and the absence of practical alternatives for policing the border, we hold that, when an officer’s observations lead him reasonably to suspect that a particular vehicle may contain aliens who are illegally in the country, he may stop the car briefly and investigate the circumstances that provoke suspicion. As in Terry, the stop and inquiry must be “reasonably related in scope to the justification for their initiation.” 392 U.S. at 392 U. S. 29. The officer may question the driver and passengers about their citizenship and Page 422 U. S. 882

immigration status, and he may ask them to explain suspicious circumstances, but any further detention or search must be based on consent or probable cause.