Severide (Taylor Kinney) and Dawson (Monica Raymund) have spent weeks investigating the suspicious circumstances surrounding Shay's death on Chicago Fire. But discovering that it was arson was just the beginning. "That's half the battle," showrunner Matt Olmstead tells "Arson is very difficult to prove."

However, that won't stop the duo from doing everything they can to try - including having Dawson call upon her brother, Antonio (Jon Seda) and the rest of the Intelligence Unit from Chicago P.D. to help. The crossover kicks off on Tuesday's new episode of Chicago Fire (10/9c, NBC) and continues on Wednesday's new installment of Chicago P.D. (10/9c, NBC). "It's our crescendo of an ending," Olmstead teases.

But will Dawson come to regret her involvement? What's next for her and Casey? And which member of the house is about to pick a fight with Severide? Get the scoop:

Exclusive Chicago Fire sneak peek: Severide comes face-to-face with Shay's killer

It looks like Dawson is put at the greatest risk in the crossover and she's already been through a lot this season. How will this episode affect and change her? Matt Olmstead: It's not as if she were an accountant by day and then this happened. It's traumatic, but also what she does for living. In a way, she's dodging bullets daily. This toughened her up for this kind of event. In our minds, what she went through is traumatic, but bringing justice to Shay's sister and to Shay's family and having the killer meet his end supersedes any kind of grief that she went through or trauma she went through.

Will this help give her closure? What is coming up for her beyond this episode? Olmstead: The big thing that's coming up for her is her relationship with Casey. From the very beginning, they've been star-crossed. They thought they had a moment where it was all coming together and it fell apart because of her desire to be a firefighter. It's this kind of dance. You can say all the right things and you can agree on all the right things with someone that you're breaking up with, who is also your boss, but the realities can be much different as we know. So it's this trick of working with, coexisting with someone you used to date and love. It's difficult for both but it turns more difficult for one than the other at any given moment. You're trying to claw your way out to get breathing room or normalcy or whatever it is. Sometimes it comes easy... or other times you have to will yourself forward, and say, "I have to go out." That's what friends are for. Certainly Sylvie Brett is coming to Dawson's aide to get her out there, but it's not easy.

Chicago Fire's Taylor Kinney talks the search for Shay's killer and Severide's "huge lesson" from her death

Is there any new romance or a possible new romance coming up for her? Olmstead: It's in fits and starts. There's a great line in one of the upcoming episodes when the Brett character says, "You've got to go out." Because Dawson sees that potentially Casey already went out and so Dawson says, "You're right." They go out, and it's not that successful of a night but she turns to Brett and says, "I need a friend right now." And Brett says, "You got one." She has that confidant in Brett, and its turbulent going forward in terms of sometime she's over it, sometimes she's not. Sometimes she's out there and she's distracted by it, and sometimes she's not.

What can you say about Casey going forward? Olmstead: It hits him the same way. He expresses himself a little bit differently. I think after the breakup, he's the first one to have a brief little romance that kind of blows up in his face. Dawson finds out about it and there's friction there. But there's no real playbook on how to successfully get out of a relationship with someone you work with. He's a little more stoic about it. There's a point where Severide sees that Dawson and Casey are kind of unknowingly going back to this dynamic of tenderness, and he tells Casey, "You might want to get out there." Casey admits, "I think I'm chasing my tail." ... It hurts him just as much as it does Dawson. He just expresses it less so than she.

Severide had a really rocky first part of the season in dealing with Shay's death. What is coming up for him in the second-half of the season? Olmstead: We have a couple things. One of them is he has a lifelong high-school friend who we're going to introduce who is kind of coming off a traumatic event of his own. And when it's time to come back to work, he brings this guy into squad to look after him and it's kind of introducing a new character to go through the group. It provides conflict because Mills has been eyeing that seat. He was in squad. In the collapse, he got injured. He had to go to ambo, but for him, it was never a long-term plan. So there's some conflict of, "That's my seat. Not your buddy's." ... After being really banged up and being patched up by other people in the first half of the season, there are a couple opportunities going forward in the latter half of the season where Severide's able to help some other people. People we know and people we're going to introduce to the show.

Chicago Fire airs Tuesdays at 10/9c on NBC. Check back Wednesday for more of our Q&A with Olmstead to get the latest on Linstead and more Chicago P.D. scoop!