Even as violent crime has declined across the country, an increasing number of police officers are being killed. According to the FBI, 72 officers were killed by suspects last year, a 25 percent increase from the previous year and a 75 percent increase over 2008. It was the highest number in two decades. It’s not clear why officer deaths are on the rise. Some say it’s because of a change in strategy: other departments are adopting the New York model of sending officers into the most dangerous neighborhoods and making frequent street stops.