Decrypt the Killer's Letters

Question

You work at a police department, and are currently investigating a serial killer who is taunting police with encrypted letters. He has promised three extremely deadly attacks still to come, and has sent 3 letters to the police that include the dates of the next three attacks.

First Letter

C D R Q F A T I U Q V I H K T J R
W O I Q V J O M G H B V W Z K H Q
C O Q V K Y L M E H P V E K E M F
O C F K C H T S M U Z E S B I V W

Second Letter

C D U A R P D D E T W B P J Z G F I
E L L U Y O W P M S L E L F X H S L
N E T M Y L O O I D V S U R U P O J
H A R O S N O Q O Q C T K V A W R P T

Third Letter

C G V I W G U C D L M E Q S
E D B O L X Z M X M J Y L Y
T H C G U C I D B P W Q T I
Z H I D I G O G J B Y A X O
W T Q H S L N A K D A Q K Y

The police profilers have come up with some details they know about the killer that might help you, but they need you to decrypt the letters to figure out the dates of the next three attacks.

1. The killer is not extremely intelligent; the cipher he is using is probably his own twist on a simple cipher.
2. The killer calls himself "The Enigma" and will always include the name somewhere in his letters.
3. The killer likes repetition; the content in the letters will likely be very similar if not exactly the same except for the date.
4. The killer will likely use a similar cipher for each letter, just modifying it slightly to make each letter harder to decode than the last.
5. The first time the killer does anything, he keeps it very straight forward. The profilers say the first character in each letter will most likely be unencrypted.
6. The formatting of the letters is not likely to be important. The killer likes symmetry and just opted to make each line roughly the same number of letters long.

So, what are the three dates of the next attacks?

Answer 1

I honestly tried only one cipher (I could say my favourite - anyone knows name or tool for that?) and it solved first part instantly:

A-Z -> 01-26
03 04 18 17 06 01 20 09 21 17 22 09 08 11 20 10 18
23 15 09 17 22 10 15 13 07 08 02 22 23 26 11 08 17
03 15 17 22 11 25 12 13 05 08 16 22 05 11 05 13 06
15 03 06 11 03 08 20 19 13 21 26 05 19 02 09 22 23

Each number is "number minus previous number", don't forget to modulo 26 (Z is 0, but I would changed it to 26), so for example 01-02 is 25 like this:
-> 03, 04-03, 18-04, 17-18, 06-17, …:
03 01 14 25 15 21 19 15 12 22 05 13 25 03 09 16 08
05 18 20 08 05 14 05 24 20 01 20 20 01 03 11 23 09
12 12 02 05 15 14 13 01 18 03 08 06 09 06 20 08 19
09 14 03 05 18 05 12 25 20 08 05 05 14 09 07 13 01

-> canyousolvemycipherthenextattackwillbeonmarchfifthsincerelytheenigma

Second part is slightly modified:

A-Z -> 01-26
03 04 21 01 18 16 04 04 05 20 23 02 16 10 26 07 06 09
05 12 12 21 25 15 23 16 13 19 12 05 12 06 24 08 19 12
14 05 20 13 25 12 15 15 09 04 22 19 21 18 21 16 15 10
08 01 18 15 19 14 15 17 15 17 03 20 11 22 01 23 18 16 20

Now you need to subtract every previous number from next number.
03, 04-03, 21-04-03, 01-21-04-23… :
03 01 14 25 15 21 19 15 12 22 …
-> canyousolv…
seems exactly same as previous message, so I skipped 40 letters from start, to just get the date, because I'm doing it by hand (I guess I should write some program for that, or find online tool)
13 01 18 03 08 20 08 09 18 20 05 05 14 20 08 19 09 …
-> …marchthirteenth si…

And third part:

is just used two times first cipher
A-Z -> 01-26
03 07 22 09 23 07 21 03 04 12 13 05 17 19 05 04 02
15 12 24 26 13 24 13 10 25 12 25 20 08 03 07 21 03
09 04 02 16 23 17 20 09 26 08 09 04 09 07 15 07 10
02 25 01 24 15 23 20 17 08 19 12 14 01 11 04 01 17 11 25

Message seems different on more parts, so I solved it all this time:
03 04 18 17 06 01 20 09 21 17 22 09 08 11 20 10 18
23 15 09 17 22 02 11 25 26 12 13 07 01 02 05 16 13
22 08 20 22 01 16 04 05 21 13 22 08 01 06 09 24 12
16 09 18 06 09 14 06 11 23 22 16 24 03 08 22 05 12 25 26

And one more time like in first cipher:
03 01 14 25 15 21 19 15 12 22 05 13 25 03 09 16 08
05 18 20 08 05 06 09 14 01 12 01 20 20 01 03 11 23
09 12 12 02 05 15 14 01 16 18 09 12 19 05 03 15 14
04 19 09 14 03 05 18 05 12 25 20 08 05 05 14 09 07 13 01

-> canyousolvemycipherthefinalattackwillbeonaprilsecondsincerelytheenigma

So to answer question:

The three dates of the next attacks are march fifth, march thirteenth and april second