MPD detectives are likely going through the key players' and any of their business ventures (like the BRC) finances. Something tells me that MPD will be looking for any recent large cash installments.
THEORY:
I can't help thinking that someone was already extorting money from SS or even AS. The hiring of this assistant, who was fired from a go kart track, who fluffs his bio, publicly shows off pics of his bosses possessions and who lies to police, might have been seen as the lesser of two evils. If said assistant wasn't getting what he wanted or getting it fast enough, it could have led to this horrible outcome. Perhaps the assistant associated with the likes of DW or DW made a point to listen in for the details.
POSSIBLE THEORY: Possibly, the assistant was always meant to get part of the $40,000 and Wint somehow knew that a large amount of money would be there for the taking. But, he could have just staged a robbery of the assistant. But, he did seem to hold a grudge so who knows... I do believe that DW somehow knew or knew of the assistant.
ETA (Edited to Add) I'm thinking some sort of blackmail was already established. If an audit is done of the Assistant and his business, it may show that his spending/purchases far exceeded any income from either his job, race proceeds, or BRC income.