Here's the indictment which has more details: https://www.justice.gov/usao-sdny/press-release/file/1449276...
They had apparently sent the following email to a large number of registered voters:
We are in possession of all your information
(email , address, telephone ... everything) .
You are currently registered as a Democrat and
we know this because we have gained access
into the entire voting infrastructure. You
will vote for Trump on Election Day or we will
come after you. Change your party affiliation
to Republican to let us know you received our
message and will comply. We will know which
candidate you voted for .
I would take this seriously if I were you .
It is interesting that their campaign seems to be pro-Trump, the guy who once vowed to tear apart the Iran deal.