(NEWSER– Police have now rounded up four suspects in the Chicago mass shooting that left 13 people injured, and they also think they have a motive. Bryon Champ, 21, and Kewane Gatewood, 20, were arrested earlier; now theChicago Tribune reports Tabari Young, 22, and Brad Jett, 22, have also been arrested and charged. Police believe Young and Champ fired into the crowd at the park, while Gatewood provided Young's weapon (a military-grade gun, perhaps an AK-47) and Jett served as lookout. Champ had been grazed by a bullet earlier that day, and went to the park with revenge in mind, say police—though a very general revenge at that.

Read the full story on Newser.com