A shooting traces back to gang-related violence in the upcoming episode 4, titled "Big Friends Big Enemies," of the fourth season of the American police procedural drama "Chicago P.D."

Lindsay (Sophia Bush) and Ruzek (Patrick John Flueger) respond to the scene where violence erupted. They learn that a car plowed through a crowd of concert goers then a shooting erupted. While the motive is still being confirmed, the Chicago PD team fears that the violence could be done by a terrorist group. So, Voight (Jason Beghe) asks for the assistance of Homeland security, customs, and the FBI.

Meanwhile, Burgess (Marina Squerciati) finds out that Atwater's (LaRoyce Hawkins) brother, Jordan (Ahmad Nicholas Ferguson), is wandering at a place where he is not supposed to. The Chicago PD team, on the other hand, decides to use all their contacts and connections in the city, in order to help them bring peace to the community and avoid more bloodshed.

The promo trailer for "Big Friends Big Enemies" shows the team racing against the clock before the next massacre happens. Before the clip ends, Voight is seen questioning who could be the key to the attacks and threatening to harm the potential asset's family if he refuses to cooperate.

In other related news, TV Line reported that actor Jon Seda's Antonio will be transferred to another "Chicago" series franchise, "Chicago Justice." The actor revealed that he is currently shooting all three "Chicago" series — Fire, P.D., and Justice. His character was first introduced in Fire. It was later moved to P.D., and now on Justice.

"It's a promotion," Seda said. "He's still a cop. He loves being a Chicago cop, and that's something that he's always wanted to be. So that's going to factor in. But moving over to the State's Attorney's office and being a lead investigator, he has his own unit there. He still gets to bring down the bad guys and help his city, just on a wider range because he'll now have state-wide jurisdiction, which he didn't have before."

"Chicago PD" season 4 airs Wednesdays at 10 p.m. on NBC.