What is ‘Stop-and-Frisk’?

Stop-and-frisk is a crime-prevention strategy that had been a staple of policing in the United States for more than 30 years before Mr. Bloomberg took office. It allows police officers to detain someone for questioning on the street, in public housing projects or in private buildings where landlords request police patrols.

Officers are required to have reasonable belief that the person is, has been or is about to be involved in a crime. If police officers believe the detainee is armed, an officer can conduct a frisk by passing his hands over the person’s outer garments.

The strategy spread with the adoption of the data-driven Compstat management system in the 1990s, which allowed the police to track and respond to crime trends in real time.

After taking office in 2002, Mr. Bloomberg oversaw a dramatic expansion in the use of stop-and-frisk. The number of stops multiplied sevenfold, peaking with 685,724 in 2011 and then tumbling to 191,851 in 2013. During Mr. Bloomberg’s three terms, the police recorded 5,081,689 stops.

“The temperature in the city at the time was that the police were at war with black and brown people on the streets,” said Jenn Rolnick-Borchetta, the director of impact litigation at the Bronx Defenders, one of the groups that has successfully sued the Police Department over the practice. “And that is how people experienced it.”

Why did Mr. Bloomberg support it?

At the same time that officers were conducting more searches as part of stop-and-frisk, crime continued to decline, a correlation that Mr. Bloomberg and his police commissioner, Raymond W. Kelly, viewed as cause and effect. The two men said stop-and-frisk was helping to take guns off the street and reduce violence across the city.

Statistics appeared to back them up: In 2002, Mr. Bloomberg’s first year in office, the number of murders in the city fell below 600, and dropped to 335 by the time he left office in 2013. Even as the amount of crime rose or fluctuated in other cities, New York’s crime rate declined, continuing a streak that had begun in 1991.