Businessman and shooting survivor David Scott is offering up the money.
Rainey was the former Marshall High School basketball star who was shot and killed in her driveway in Maywood in July.
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There have still been no arrests in the case.
Rainey's mother hopes the money will entice someone to come forward.
The reward announcement was part of a bigger "Peace Rally" event in Maywood.
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Several doves were released as a sign of peace and hope and to remember those who have been lost to violence.