One Tree Hill alumna Sophia Bush and Patrick Flueger (The 4400) have joined Chicago Fire‘s upcoming spinoff series Chicago PD as series regulars. On the NBC midseason drama, about the workings of the Chicago police department’s investigative team, Bush will play Erin Lindsay, a member of the team with an unexpected past. Flueger will play Kyle Ruzek, a hotheaded rookie plucked by Voight (Jason Beghe) from the Academy for dangerous, delicate undercover work for the Intelligence unit. The two join fellow CPD stars Beghe and Jon Seda, who were in the planted spinoff episode of Chicago Fire, and Jesse Lee Soffer, who joined earlier this summer. Bush is expected to fill the female void on the show left by the exit of original female lead Melissa Sagemiller, who was not picked up beyond the backdoor pilot. Dick Wolf, Matt Olmstead, Danielle Gelber, Peter Jankowski, Michael Brandt, Derek Haas and Mark Tinker executive produce for Wolf Films and Universal TV.