In New York, Tsarnaev’s mosque quarrel and his sudden behavioral changes might well have been reported by concerned worshippers, the imam himself, or other fellow Muslims, given the NYPD’s close ties to Muslim preachers and community leaders, as well as its network of tipsters and undercover operatives. Once it had Tsarnaev under surveillance, the NYPD, through its sophisticated cyber-unit, might have detected his suspicious online viewing choices and social-media postings. Other detectives might have picked up his purchase of a weapon, gunpowder, and even a pressure cooker—an item featured in an article, “How to Build a Bomb in the Kitchen of Your Mom,” in the online al-Qaida magazine Inspire. Even if the NYPD hadn’t been watching Tsarnaev, it might have been tipped off to such suspicious purchases thanks to its Nexus program, launched in 2002, under which the department has visited more than 40,000 businesses in the metropolitan area, encouraging business owners and managers to report suspicious purchases or other activities potentially related to terrorism.

If Tsarnaev and his brother had still managed to evade police scrutiny, it’s possible, though not likely, that the NYPD’s “Ring of Steel”—its network of 4,000 sophisticated security cameras, which feed information into a central monitoring system to detect questionable or unlawful activity—might have enabled officials to notice the pressure-cooker bombs before they exploded. Though the department has focused its camera network so far on the Financial District in Lower Manhattan and on such iconic sites as the Empire State Building and Grand Central Terminal, at least 220 cameras have been installed with views of Central Park, where the New York Marathon reaches its finish line.

Finally, the NYPD’s early effort to understand Muslim communities and follow tips and leads by sending plainclothes officers to mosques, restaurants, and other public venues where Muslims congregate—in keeping with court-ordered guidelines governing such surveillance—might have secured information preventing last week’s bombings.