CHICAGO — In a city wrestling with a rise in gun violence and turmoil in its police department, August seemed like the longest month. By midnight on Wednesday, 90 murders had occurred in August alone, making it the deadliest month in Chicago in about 20 years.

Chicago, the nation’s third-largest city, has experienced more homicides this year than the bigger cities of Los Angeles and New York combined.

The number of deaths here this year, 471, is about 50 percent higher than the same period a year earlier. Nonfatal shootings have risen at a similar rate. More than 2,300 shootings have taken place so far this year, mostly on the South and West Sides and most of them involving African-Americans.