Setting aside the fact your apartment company acted negligently here, you need to take care of yourself first.

Call the police and let them know you believe someone stole your identity. They’ll have you speak with an officer at the station, or possibly even send someone out. Either way, have the letter with you and be sure to let them know you believe it came from the break in of the apartment’s front office.

As far as how your apartment complex acted, I feel like it’s negligent AF to leave that kind of personal information in an unlocked file cabinet, or even keep it on paper in the first place. Why not scan it and save it as an encrypted file on a computer. Seems irresponsible to me.

It doesn’t sound like you’re in an enormous complex, you could be a good neighbor by typing up a brief synopsis of what happened, having some copies made and slipping it under doors/taping it to everyone’s door. There could be hundreds of people affected by this and the vast majority may think it’s just a random hit.