[El Gato]

I don't think there's any concrete evidence here that there are two people involved

Of course there's not much concrete evidence period. There is at least some eyewitness evidence that there are two people involved. It's probably not very reliable, but the same sort of evidence as that indicating "white box truck", "white mini-van"

[AntiGuv]

I've heard that there might not be any white van directly involved at all. It's been rumored in the area that for whatever reason - either to throw off the cops or as part of some weird ritual delusion - he simply waits to see a white van before he shoots a victim. That way, everyone's looking for whatever white van happens to be in the area rather than wherever the killer might really be at.