At least 12 major U.S. cities have broken annual homicide records in 2021 - and there's still three weeks to go in the year. Of the dozen cities that have already surpassed the grim milestones for killings, five topped records that were set or tied just last year. Philadelphia, a city of roughly 1.5 million people, has had more homicides this year (521 as of Dec. 6) than the nation's two largest cities, New York (443 as of Dec. 5) and Los Angeles (352 as of Nov. 27). That's an increase of 13% from 2020, a year that nearly broke the 1990 record. Chicago, the nation's third-largest city, leads the nation with 739 homicides as of the end of November, up 3% from 2020. Experts say there are a number of reasons possibly connected to the jump in homicides, including strained law enforcement staffing, a pronounced decline in arrests and continuing hardships from the pandemic. According to the FBI's annual Uniform Crime Report released in September, the nation saw a 30% increase in murder in 2020, the largest single-year jump since the bureau began recording crime statistics 60 years ago. The retirement rate in police departments nationwide jumped 45% over 2020 and 2021. And another 18% of officers resigned