Well, we have two airports here in Vegas, this is true. One is McCarren Airport, which does all the commerical flight, as well as another one, which I forget the name, which mainly is for people who have their own aircraft, like sesnas, and shit like that. (Occasional helicoprters...ect.). The only other place would be Nellis Air Force base, where jets take off.
I am fairly acquianted with the patterns of flight from McCarren. (I live pretty close.) Now, I have been paying pretty good attention, and the McCarren planes are not the ones leaving the suspicious contrail. Everything appears normal with the commercial side of things. Now, the criss-cross pattern is only suspicious becuase it all gets laid down in the matter of three hours. (not over the course of the day). It wouldn't be suspicious if the pattern appeared slowly, but all at once the planes hit the skies and start the pattern. You can watch one plane fly one way, and watch two other planes criss-cutting his lines. They do it all in one go.
Now, I have no way of measuring their altitudes myself. The conditions would have to be right for their contrail to "linger", and from what I can see, planes with comparable altitudes do not leave contrail which lasts as long as the jets I have been noticing. (The ones that come in together and lay the grid formation down). I am trying to gather more information, but it is kind of hard gathering solid information about these activities.
the local news thinks its suspicious too, but there is no evidence to say that it is harmful, so they are not going with anything. (a wise choice). I just wonder why it happens, its baffeling.