The whole department: John, Jeff, Jean, and the accountant [?] were thrown into jail.
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The colon is incorrect here.
Between two main clauses in cases where the second clause explains or follows from the first:
To introduce a list:
Before a quotation, and sometimes before direct speech:
Your example is a list, but it should not have a colon because the opening words are separated from their verb. You could rewrite it as:
Commas are fine where your list items are short. However, there might be times where semi-colons are more suitable, to aid clarity: