Spy on the Submarine

During the Cold War, a submarine communications officer received the following message in morse code which, when decoded, read:

TOP SECRET
Captain Mitchell
We have a spy in our midst and we believe he
is giving information to the Russians. We
have recently made a breakthrough and have
been able to narrow down our list of suspects
to one of the 252 members of your crew. We
have no exact information and no physical
description. We are convinced that you are
not the spy and as such we entrust you to
perform some covert investigation to aid us
in our attempt to catch the rogue operator.
We wish you good luck in this endeavour but
we must remind you to keep this secret. We
needn't remind you a refusal to follow these
instructions exactly to the letter will
result in an immediate courtmarshall.
Instructions for arrest:
1) If you fail to find the spy then, upon
docking you will ensure that none of your
crew can leave the ship.
2) If you do ascertain who the spy is then
you should arrest him, confine to the Brig
immediately & place him under 24 hour guard.
From this point on you should cease all
communications with us.

Naturally the Captain was purturbed by this message. He was just about to destroy the message to ensure safety when he was presented with another message which read.

Addendum to previous message:
D-C / T-H / A-A / O-H / C-A / X-K / W-A / O-F / X-D.

After the Captain read this message he was a little less purturbed than he was before and soon decided on an appropriate course of action. What was it?

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