The previous highest monthly homicide tally in Chicago was in August 1996, the Chicago Sun-Times reported citing historic Chicago Police Department data.
It did not provide the number of deaths. There were 384 shooting incidents and 472 shooting victims during August 2016, police said.
Chicago is on pace to have more than 700 homicides by the end of the year, the newspaper reported.
The city hasn't had more than 600 homicides in a year since 2003 and not more than 700 since 1998, according to records.
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"The historical cycle of violence we have seen in some communities must come to an end," CPD Superintendent Eddie Johnson said in a statement.
"Repeat gun offenders who drive the violence on our streets should not be there in the first place and it is time to changes the laws to ensure these violent offenders are held accountable for their crimes."
The 90 homicides for this August compared to 54 killed in the same month last year. Police say they made 18 percent more murder arrests this past month compared to August 2015. The department's number of gun arrests this year is up 5 percent compared to the same time during 2015, it said.