Anything and everything below this section is SPOILERS for the late game plot. Do not read any further unless you have completed case 5.

In order to identify 404, we have to get some clues together. The first clue is one they give themselves: we have "met" them during these cases. Assuming you've scrounged around for clues along the way, and thus have gotten all the clues in the game (including bonus clues), we're in a good place to begin our search.

In case 5, we have learned that Sun Zhi was an agent of 404's from Song's journal found in his safe. We also learned he always used the password 112811 from his wife's testimony on the combination lock to their front door. According to Song's journal, Sun Zhi was going to sell him the identity of 404 by providing some photographs of 404, hidden encrypted in a photograph. The only clue to its location being "No. 12". With Sun Zhi dead, this is the only clue we have. Thankfully, there's only one place in the game that actually has a number 12 in it: The Chicheng Library. Combine that with the passcode (112811) and we don't get the photo, as 404 has already taken it. However, he leaves us a clue: the case file number for Case 3. Since nothing in Case 3 is relevant to the Case 5, it must be relevant to 404's identity.

In addition to locker 12, there is another locker we can open. According to Nie Ping, he once saw two guys fighting over a storage locker, with one of them being a journalist named Zhang. If you recall, we know a Zhang from case 1: Zhang Feixiang. What's more, in case 1, Zhang has a notebook. Containing two passwords. You could [if you're boring] brute force at this point. However, if you want to go further, we actually know who Zhang was fighting. In case 2, Huo Yan testifies that he got in a fight with a guy in the library, just after opening locker number 7. Combining these, we can deduce that we can open locker 7 using one of the two passwords in Zhang's notebook. Doing so gives us another notebook, containing more information on 404. From this, we learn that Song hired Zhang to investigate 404. We learn that 404 had another agent aside from Sun Zhi, likely within CCPD. Considering they have an employee number for Black Robe (which worked on CaseCracker), this is likely how 404 got their "in" on CaseCracker, and has been hacking you.

A final two clues can be obtained in Case 5 from the Sphinx app. Hao Ran says that an infamous cultist named Sphinx was an IT guy who had a son or daughter. Considering the close relation 404 has with Sphinx, we can assume this child is 404. This means that 404 would be an only child with a father in IT

Clues so far:
- Something to do with Case 3
- Not a hacking expert themselves
- An only child
- Father was an IT expert

Speaking of Sphinx, if we head over to Project K in case 5, Hao Ran has more information for us. Turns out that Project K was built by Sphinx, which means that it's likely 404 is involved in it. There is a single database that can only be accessed with specific passwords. The clue to access them comes from the website address. CC.CASEID.K. In other words, we can unlock database entries with case ID's. Happily, we've got five of them from the cases we've done. Plugging them in and ordering them by date, we get the phrase "Pay Attention To The Logo". If you've been paying close attention, you will have noticed that the Sphinx app always has a logo, and that it differs each time. The Sphinx app appears in cases 2 to 5, giving us four logos. These are the Gemini zodiac, Shiva the Hindu deity, a three-legged golden toad, and a mouse/rat. Searching these terms on the relevant cases gives us a bit more detail. From these, we can deduce:

- Gemini is a sign that is designated to people born between May 21st and June 21st. Since the English version of the game uses USian dating, this means that we're looking for someone with a DoB between 21/5 and 21/6.
- Shiva can be both benevolent and fearsome, but is well known for their... manliness. We're looking for a man who can be both kind and cruel.
- The Golden Toad - brings with it wealth and prosperity. We're looking for someone who is either rich, or makes other people rich.
- The Rat/mouse hides away, and is sneaky. We're looking for someone devious who avoids direct confrontation. It is also one of the Chinese Zodiac animals, and as the game is set in China we may be looking for someone born in the year of the rat. Recent ones are 1972, 1984, 1996, and 2008 ( thanks, Wuya666).

After you've gathered together the evidence, you can start hunting through each case for people. It's easiest to grab profiles and find Geminis first since DoB is given for all people. From there, we can remove poor people, honest people, women, and anyone with a confirmed sibling. From my own deductions, I ended up with Zhu Xixiang, and Gao Jingwei. Let's take a closer look at these two men.

Zhu is a real estate developer who suddenly became rich in 2013 (based off age and DoB), a couple of years after the 517 incident. Nobody knows how. He uses bribery and manipulation to get his way, and is generous to his staff and those who please him. Whilst powerful, he is not particularly dangerous. He prefers the carrot to the stick, and treats his employees fairly. He has the Sphinx app installed on his computer.

Gao is an MIT graduate and CEO of Peco Therapeutical. He is an egotist who takes a lot of care of himself, and always seems to know more than he lets on. He is a chronic womaniser. He is fiddling his expenses, and tends to be dishonest. He also has the Sphinx app on his PC.

Both fit the bill quite nicely, and I'd even argue that Zhu is a more likely suspect. However, this is discounting the clue from locker 12, which refers directly to Case 3. Not only is this the case with Gao in it, but it also has numerous strange elements. The culprit (Chen) got the murder idea off Project K, and followed it through. However, he himself has no idea how the knife ended up in the statue base. Additionally, a figure in a blue denim jacket is spotted on camera putting the photo of Wang Feng in its pocket. The only person who knew the shirt was here was the person who told Chen to put it there on Project K. The denim jacket was later sent to Li Yiying, sent by 404. This means that 404 is definitely active on Project K, and that they were directly involved in planting evidence in this case. The choice to give the denim jacket to Li was not incidental, as her father was directly involved in incident 517, where Sphinx was arrested. To conclude, 404 takes a direct and personal role in Case 3, unlike every other case. Finally, referring back to the rat, one of these two man just so happens to have been born in one of the years of the rat previously mentioned...

Given all the above information, we narrowed it down to 2. Given the final clue and some logic, we can narrow it down to one.
Spoiler just in case you don't want to catch it by accident...
Gao Jingwei!!!!!!!!!!!!!!!!!!
Given this is the only case with direct involvement from 404, his repeated, clear dishonesty (including claiming he doesn't know Li), and the fact he otherwise fits the bill perfectly.

Hope this guide has helped you understand how you're supposed to determine the identity of 404.