Dear …
I want to take this opportunity to say how impressed I was with the way __(you/you and your partner/your whole department was/etc)__ were able to bring order back to __(our neighborhood/my life/etc)__ by __(briefly explain what the police officer did)__. __(I was/We were all)__ very frightened by the way those __(type of people)__ were behaving and felt that __(my life/our lives)__ were in danger. It was an experience __(I/we)__ would not want to live through again.
You can’t know what a tremendous relief it was to __(me/us all)__ to learn that you had __(captured the perpetrators/etc)__. And now, __(I can /We can all)__ sleep easy knowing that __(I am/we are)__ safe, once again. __(I/We)__ just wanted to extend a personal thank you to __(you/you and your partner/your whole department/etc)__ for the dedication and courage you __(all/both)__ bring to your job(s). You put your __(life/lives)__ on the line every day to protect citizens and __(I /we)__never realized, until now, how grateful __(I really am/we really are)__ for that.
Once again, thank you for your quick response in bringing safety back to __(my life/our lives)__. The kind of dedication and bravery you __(all/both)__ demonstrated __(today/last night/last week)__ will never be forgotten.
Sincerely,