They'll board the "Chicago Fire" spinoff set for midseason on NBC.

NBC's Chicago Fire spinoff is adding a pair of new regulars to its ranks.

One Tree Hill alum Sophia Bush and The 4400's Patrick Flueger have joined the Dick Wolf drama as series regulars, The Hollywood Reporter has learned.

The spinoff, which aired as a back-door pilot during the freshman run of Chicago Fire, takes place at the Chicago Police Department.

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Bush will play Erin Lindsay, a member of the CPD investigative team with an unexpected past. Flueger, meanwhile, will play Kyle Ruzek, a hot-headed rookie plucked from the academy by Voight (Jason Beghe) for dangerous, delicate undercover work at the intelligence unit.

The duo join other Chicago PD regulars including Jon Seda and Jesse Lee Soffer on the midseason drama from executive producers Dick Wolf, Matt Olmstead, Danielle Gelber, Peter Jankowski, Michael Brandt, Derek Haas and Mark Tinker.

The series hails from Wolf Films and Universal Television.

Bush most recently was attached to co-star in NBC's Hatfields & McCoys drama -- where she would have co-starred with Soffer and Flueger.

Bush is repped by WME, Joan Green Management and Jackoway Tyerman; Flueger is with UTA and Nancy Kremer Management.

E-mail: Lesley.Goldberg@THR.com

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