When a new murder happens, you get a call pretty fast.
If possible, you should rush to the scene immediately.
The murderer can, and often will, be in the vicinity.
At the very least inspect every person inside and leaving the building, and pin them to your board.

Occasionally you can squeeze in asking 1 to 3 people if they've seen anything suspicious, and they'll just happen to describe a guy next to you.

Here you are looking for 10 things, You will be pinning things in mass. Everything except duplicates of the same things belong on the board. You want to be thorough.

• Establish who the victim is (Pin him.)
  
• Take his fingerprints (Pin them.)
  
• Take his boot size (Pin it.)
  
• The time of death & the type of the murder weapon (Pin it.)
  
• Building he's in (Pin it.)

Prints will be scattered all around the apartment, these will include the victim's, (optionally) the victim's spouse, and the murderer. [Enforcers will NOT leave fingerprints.]
Frequently they will touch doors, lights, the safe, or notes the murderer left behind. Check any drawer that is open.
In corporate settings there might be more. In which case you'll have to rule out every co-worker. In that case you can go through the employee database and get prints of every employee.
[As far as I am aware, the murderer must leave a minimum of 1 print? But they can be in nearly impossible to find spots. Such as ontop of curtains, inside drawers etc.]

Footprints of the victim, murderer, (optionally) spouse, and often the enforcer's footprints will be scattered about. Pin them all.

Often the murder weapon will be on the crime scene. Wherever its just laying loosely on the floor, in a drawer, on the open on a counter, or even hidden behind a pillow. You want to look for any type of weapon that matches the murder weapon, and scan them for prints. When you find it, pin it.
[You can also do so from your inventory screen.]
[Note for devs: It should be possible to match a weapon to the murder, even without prints. Currently it isn't.]

Every household has an address book. Inspect it. This is a potential goldmine. Pin it.

You're going to find the victim's place of employment through a worker ID, wallet, employment contract in their closet. Pin the workplace.

The victim's computer sometimes has mail with hints to the murderer. If you see anything suspicious, print it. Further, print any clubs they go to. It all needs to be pinned.

Go to the telephone and see if a call was made, if so note it in your casebaord. You can find out which building the call came from, from the telephone box in the basement.

Notes can reveal if the victim was doing something else. Such as shopping at a particular place, getting appointments somewhere. These things need to be pinned as well. The victim's landlord also should be pinned, from their contract.

Occasionally the murderer will leave writing on the wall. Photograph it. Pin the Photograph

In this particular example I managed to ask witnesses outside the building for hints before entering it.
A thorough investigation of the victim's house left me with plenty of leads.
Security footage is not available, as it did not spawn in the apartment building, making this case more complicated.

Security footage has a 24 hour lifetime. If you're too late, footage will be gone.
Every building should have a security room [To Do: Verify. I've seen some bugged ones.], but it can spawn in awfully random locations. In apartment buildings they can spawn on completely random floors, and there is no way around simply running through the entire stairway and checking every door. However the security room will have an obvious camera icon on it, and be marked on your map once you find it.

From the estimated time of death, add an extra hour to both ends of the estimates, and check the footage. What you want to do is to print out photos and pieces in mass, pin them to your board.
Once you have the timerange covered, you can click on the photographs, click on connections and add the potential suspects from the photographs to your board.

Its time to knock on the doors of neighours of the same floor, the ground floor and random people on the street if they've seen anything suspicious. Ideally, they will have and add clues, such as the build of the victim. Note it in your case-board.

They can also identify people from photographs, which is why you have suspects from the security footage pinned. You can ask about them, and occasionally they will give you the name of a suspect.
People typically know their neighbors, if somebody stands out as not known by anybody, he becomes a prime suspect. They're not neighbours, or usually show up here, then.

At this point you should have all the material for a successful investigation. Its time to go to the caseboard, and to draw some lines and collect the data you have on the suspect. Prints, boot size, details from witnesses, calls, notes, photographs. You'll always have something to work with.
Also click on the address book and pin all their friends. They're suspects now.
Its time to rule out suspects. Simply cross-reference details you know about the murderer, and cross old suspects off by right clicking the pin on their note.
You'll be able to eliminate plenty of people fast, and have tons of open threads to work through. Normally, you'll be able to leave only a handful of suspects whom you can work through one by one.

Ideally you already have solids leads that you are pursuing, otherwise its time for plenty of legwork and closing out leads.

If you have fingerprints, its relatively easy to rule them out. Simply go to their front doors and scan their doorhandles. You'll be able to rule out the entire building in that case.
[To do: Can you check out residents fingerprints from the security room? Double check]

Go to their place of employment, either break in or buy a guest pass. Get a list of all the employees (usually they have a board with all of them), enter the employee database. Get info on every single employee. You'll have their names and prints. This is going to allow you to rule out a lot of people.

From the victim's address book, pin all their friends

If the victim had a phone call, you know which building it came to, go to that one, open up the telephone box and see which apartment it was. Investigate and rule out said caller.

People can be members of a club, they will meet every once in a while in a restaurant or bar. Typically they're advertised in Newspapers, and sometimes in the restaurants themselves. Restaurant computers will also have emails about these events.
[To do: Find out more about this. Currently its really rare and incredibly hard to find out about this.]

The victim probably had random receipts lying around from places they've been to. Restaurants, bars, clincs. Its time to hit them, get the employee records and rule out suspects from those places.

This is the place nobody wants to go to. When every good lead fails, you can't rule anybody out, or everybody has been ruled out, and you're left with ♥♥♥♥-all.
Its time to hit the government database on the third floor of city hall. Get access to a computer, enter a suspects name and you'll get almost all their info. This is borderline cheating, and works extremely well.
More than likely you miss something vital, or the case is simply incredibly hard. Either can be true. Re-check your work.
If even that fails, there is only one thing left to do:

Sit back, and wait.

The murderer will strike again.

a. Security rooms can spawn on random floors within the buildings, or fail to spawn. [Dev note: Is this intentional?]
b. The homeless refer to other homeless as 'living with them' which can be quite confusing
c. The workplaces of street vendors are set to the building they live in, which doesn't make much sense.

This concludes this guide! It'll be updated and filled out more in the future.