Police Superintendent Garry McCarthy made the announcement prior to a march from Pendleton's school to Harsh Park, where the 15-year-old was gunned down last week in what was believed to be a gang-related shooting resulting from mistaken identity.
Pendleton, 15, was a member of the King College Prep marching band, which took part in last month's presidential inauguration parade in Washington. Police believe she and some friends were mistaken by the shooter for associates of a rival gang, the Chicago Tribune said.
There have been few leads in the case and police and community leaders said they hope the combination of public pressure and financial gain will generate a tip that will crack the case.
"If you know who did this, turn them in," a leader of the march called on the crowd. "If you don't support this, next it might be you."
Demonstrators urged the city to increase the size of the police force.