A police officer in the Bronx said that officers detected a mixed message from the top. In recent months, the officer noted, Mr. Bloomberg’s support of the stop-and-frisk tactic has, in the eyes of many officers, softened. In addition, the officer noted, Mr. Kelly ordered all officers to go through retraining on how to conduct lawful stop-and-frisk encounters. “You see an about-face, and it’s like they’re saying there was something wrong with how it was done before,” the officer said. “Before, you had the mayor and the police commissioner defending stop and frisk to the end.”

The officer also noted that a federal-court decision in May granted class-action status to a lawsuit on behalf of many New Yorkers who had been stopped. That decision, the officer said, had concerned officers, leading them to wonder if the federal judge was ultimately going to hold that the Police Department’s street stops had led to widespread Fourth Amendment violations.

“People read those articles and realized this may be illegal,” the officer said. A Supreme Court decision in 1968 permitted the police to conduct street stops if they had a reasonable suspicion that criminal activity was afoot and to subsequently frisk the person if the officer was concerned for his or her safety because of a belief that the person was armed.

In discussing the falling numbers on Friday, the mayor’s chief spokesman, Marc La Vorgna, suggested that the mayor thought the stop-and-frisk process needed to be fixed. “As the mayor said, we needed to mend, not end, the practice, and the reforms Commissioner Kelly has put into place ensure the focus is quality, not quantity,” he said.

The number of street stops has increased every year during Mr. Kelly’s tenure, except in 2007. After the first quarter of 2012, police and city officials began to wonder how high the number would go in future years, and some privately questioned why the number of street stops continued to rise even as crime levels remained relatively flat in recent years.

But the drop over the past three months, whether caused by officers’ skepticism, Operation Impact changes or a combination of both, could also suggest that the practice of stop and frisk may have crested in early 2012 and may now be on the decline.

“I do believe there is a realization on the part of the New York Police Department that perhaps the number of stops got too large for the communities and the police officers to deal with,” the chairman of the City Council’s Public Safety Committee, Peter F. Vallone Jr. said. Mr. Vallone, who is generally supportive of the police’s use of street stops, said that the number of stop-and-frisk encounters had gone “higher than we should go,” given a reduced police force. He added that “officers had been left feeling the strain of a large amount of stop and frisks.”