How does McCain know that illegal aliens are the ones setting up fires to dry brush in Arizona?

It's common for illegal immigrants to set forest fires in the west. The particular area is known to be used by illegals and in those areas they often build fire to keep warm, but on occassion they get out of control. I have not seen what evidence there is, and it would take time for us to fully investigate the fire, but even if illegals did not physically set the fire, they are still partly to blame. The west is dry, but illegals make it worse. adding millions to the south west has reduce the water table and has made our forest drier and more vulnerable to forest fires.