In The Great Gatsby, why does Tom think Gatsby is a bootlegger?

After the first of Gatsby's parties to which they are
invited, it is clear that Tom begins to feel threatened by Gatsby and his lavish wealth.
He makes clear his intention towards the end of Chapter Six to look into Gatbsy and how
he so quickly gained such wealth and prestige. In Chapter Seven, it is clear that he now
has that information, which he uses as part of their argument over Daisy whilst in the
hotel during the blisteringly hot day. Tom says to
Gatsby:

"I've
made a little investigation into your affairs--and I'll carry it further
tomorrow."

Thus it is that
the investigator he has paid has told him precisely how Gatsby has made his wealth, and
it has been by working with Meyer Wolfsheim and buying up lots of drug-stores where they
sell grain alcohol over the counter.