It was an unusual forearm tattoo that the police said led them to Luis Reyes, a 35-year-old man who was accused of stealing packages from a Manhattan building's mailroom in 2019. But the truth was more complicated: Mr. Reyes had first been identified by the New York Police Department's powerful facial recognition software as it analyzed surveillance video of the crime. His guilty plea this year ... was part of the sprawling legacy of one of the city's darkest days. Since the fall of the World Trade Center, the security apparatus born from the Sept. 11 attack on the city has fundamentally changed the way the country's largest police department operates, altering its approach to finding and foiling terrorist threats, but also to cracking minor cases like Mr. Reyes's. New Yorkers simply going about their daily lives routinely encounter post-9/11 digital surveillance tools like facial recognition software, license plate readers or mobile X-ray vans that can see through car doors. Surveillance drones hover above mass demonstrations and protesters say they have been questioned by antiterrorism officers after marches. The department's Intelligence Division, redesigned in 2002 to confront Al Qaeda operatives, now uses antiterror tactics to fight gang violence and street crime. The department's budget for intelligence and counterterrorism has more than quadrupled, spending more than $3 billion since 2006, and more through funding streams that are difficult to quantify, including federal grants and the secretive Police Foundation.