Learn English – What punctuation should follow after a list introduced by a colon

The colon is incorrect here.

There are three main uses of the colon:

Between two main clauses in cases where the second clause explains or follows from the first:

• That is the secret of my extraordinary life: always do the unexpected.

To introduce a list:

• The price includes the following: travel to London, flight to Venice, hotel accommodation, and excursions.

• The job calls for skills in the following areas: proofing, editing, and database administration.

Before a quotation, and sometimes before direct speech:

• The headline read: ‘Taxi Driver Battles Gangsters’.

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:

The whole department was[not were] thrown into jail: John, Jeff, Jean, and the accountant.

Commas are fine where your list items are short. However, there might be times where semi-colons are more suitable, to aid clarity:

The whole department was thrown into jail: John, who had been with the firm for forty years, had always been highly regarded and had never missed a day through sickness; Jeff, the boss's blue-eyed boy who until this incident had done no wrong; Jean, who everyone knew had money worries and therefore might have been tempted to steal; and the accountant, who nobody had ever liked anyway.