The phone number (that shows who received the message) is important.

Examine the offender record carefully.

This message is encrypted using a Caesar cipher with two keys. That means every 1st, 3rd, 5th etc. letter is shifted by one number (the first key), and every 2nd, 4th, 6th etc. letter is shifted by a different number (the second key).

For example, if you wanted to encrypt the word "FACE" using the following keys: (1,2), you would shift the 1st and 3rd letter by 1 (i.e. F—> G, C—> D), and the 2nd and 4th letter by 2 (A—> C, E—> G). Your decrypted word would be: GDCG.

The message was sent to Marcus Calden (the phone number on the messages matches the phone number listed on the offender record). What gang is Marcus in?

This message is encrypted using two Caesar cipher shifts. Every first letter has been shifted by eight, and every second letter has been shifted by nine.

This message was sent to Marcus Calden (the phone number matches the number listed for Marcus on his offender record), who is in the Eight Nines gang (which is why 8 and 9 were chosen as keys for encrypting this message).

To decrypt the message, shift every first (and 3rd, 5th, 7th etc.) letter backwards by 8, and every second (and 4th, 6th, 8th etc.) backwards by 9.

Decrypted, the message reads:

ITS DONE.