Absolutely. Just as soon as firefighters have secured enough of a site, investigators get into purported ignition areas, well before media or people are allowed, even as the remains are smoldering. They have to get to valuable information - what was there at the ignition point? were there any traces of accelerants? where there ignitables stored there? etc. They have to get access to sample for all sorts of chemicals or such before more water, or rain, or foot traffic obscures significant details. They know what they are looking for.
During the active firefight, the brave fighters can often spot clues about the most suspect sites and lead investigators to the best sites of inquiry.