How do I appease the cops?

So, things didn't go exactly smoothly during my last visit to the police station, and everyone inside is dead except me. Now, whenever I walk out the front door, I am very quickly put down by a few cops who happen to be standing outside.

How can I get them to not attack me on sight? Are they feeling vengeful because I slaughtered their coworkers, or is it just because I have my weapon drawn and don't know what button will put it away?

I do know about an alternate exit, but that's a pretty long detour and I'd rather just use the front door if at all possible.

You can holster it on the Xbox 360 by pressing Y or △ on the PlayStation 3. On the personal computer version it is H. Alternatively, you can un-equip it in the inventory screen.

However this is unlikely to help as you are probably be marked hostile by the police. There are 2 other doors you can take to get out. I would recommend you take the alternate exit and see if later in the game (if this is the first time in Detroit) the police will be less hostile.

If you have truly upset the police, you're going to be spending a decent period of time avoiding them as they're going to agro you on sight.

Try exiting the police station via the sewers instead of the front door, you need to hack the door on the basement level where the autopsy room is to get out via the sewer, the loading screen into that part of the zone never has any enemies immediately after the loading screen.

The other exit is on the second floor of the police station, and leads to a fire escape on the side of the police station. This doesn't open up into completely clear space (if you remember the O'Malley mission, the garage you break into is in this area of the game). There should be no police officers directly in line of sight of you as you exit though.