another phrase to express dwelled on

After the incident he dwelled on the issues surrounding the incident.

What would be other substitutes to express the idea conveyed by "dwell on?"

I considered "mull over", "ruminate", "moped about", etc., but these terms don't communicate the thinking at length about something negative, a frustrating aspect of the term "dwell on." Some of these are too formal or don't have association with the negative frustrating nature of the event.

Brood would work perfectly with what you're describing because it implies both length and negativity. Merriam Webster defines it this way:

Brood (verb): to think a lot about something in an unhappy way

Perhaps chewed over

(transitive, adverb) to consider carefully; ruminate on

Collins

Also agonize (usually with over)

Undergo great mental anguish through worrying about something

Oxford Dictionaries Online

Check if lingered (over) appeals to you.

After the incident he lingered over the issues surrounding the incident.