Mystery about disguised murderer leaving behind hats

Can someone help me find/name a specific movie, please? Details:

• mystery, maybe thriller;
• probably 2000s or possibly 90s;
• about a mysterious murderer leaving behind female hats at the scene;
• the protagonist (good main character) is named Susan, I believe;
• it turns out the murderer is not a female, but her romantic partner.

Any help would be greatly appreciated!