It contained blood. First, they used something like luminol (different brand, it's in the PCA). Then, when those luminol (already invisible) prints faded, they used Amido black which enhances proteins found in biologic material, so that it helps to pick up MORE invisible footprints that the luminol missed.
When a person steps in blood, typically the first couple of steps are visible (or, more, depending on amount of blood). Then the next steps are invisible to the naked eye but visible to Luminol (or similar). Then, when those too disappear, bring in the Amido black. (
Amido black 10B - Wikipedia )
Latent means "not visible to the naked eye."
Latent Evidence
Footprints are not left by anything but a foot, whether in a shoe (as with this one) or not. Human footprints are distinctive and can give markers as to what the foot inside the shoe looked like.
So,
since the use of Amido black only works on biologic proteins and since Luminol had already picked up other latent prints in blood, they looked for more bloody footprints. And found them.
So unless they had a party AFTER the four students were killed, leaving blood in the house, the footprints came AFTER the blood-letting.
Obviously, they know exactly whose blood was on the bottom of the shoe (I predict it will be Xana's, given what we know).
And that's how they know that the print was left in only one way. If it had not been blood, there would have been no point in using Amido black and/or luminol for those latent prints.
There is no "somehow," to me about how the blood got out of the people's bodies and onto the floor, to be stepped in. But that's how there came to be a latent print. Prints made with water do not show up with Amido black or luminol. Only blood proteins show up with luminol (after enhancement with Amido black).
IMO.