Horizontal collective knowledge in Massachusetts requires the officers communicate with each other and share information. Commonwealth v. Privette, 2023 Mass. LEXIS 86 (Mar. 28, 2023):
We conclude that, with respect to the horizontal collective knowledge doctrine, art. 14 requires more. To be consistent with the requirements of art. 14, in order to aggregate officers’ knowledge, the officers must be involved in a joint investigation, pursuing a mutual purpose and objective, and they must be in close and continuous communication with each other about that shared objective. While the officer who actually effectuates the stop need not have personal knowledge of all of the officers’ pooled knowledge giving rise to reasonable suspicion or probable cause, the officer must be aware of at least some of the critical facts and must have been in communication with others who have such knowledge.