In other words, what constitutes a legal stop (seizure), and search or pat-frisk, according to Massachusetts law? This may be confusing for many people. Citizens have an expectation of privacy and want to avoid harassment or unreasonable search and seizure, yet clearly the police have to be able to protect themselves and the community from crime. The law in this case, as so often, is about trying to achieve the right balance between two equally important goals.
Terry v. Ohio, 392 U.S. 1 (1968) is the seminal United States Supreme Court case that sets the standard requirements for a stop and frisk. The term “Terry stop”, commonly used in legal parlance, derives from this case. According to the Terry case, there is a 2-pronged requirement for a stop and frisk.
The recent case of Commonwealth v. Narcisse, 457 Mass. 1(2010) clarifies the standard in Massachusetts. "This court concluded that police officers may not escalate a consensual encounter with an individual into a protective frisk absent a reasonable suspicion that the individual has committed, was committing, or was about to commit a criminal offense, and that the individual is armed and dangerous."
One of the best sources for researching this question is Suppression matters under Massachusetts law (by judges Joseph A. Grasso & Christine M. McEvoy, updated annually by the publisher Matthew Bender), particularly Part II (chapter 4 & 5): “Stop and Frisk: a form of warrantless search and seizure.” This discusses warrantless searches of people both on foot and in motor vehicles.