In this case, the flashback takes place in the middle of the first scene, so I didn't want to break it into two chapters. Basically, Tom is sitting in his car in a traffic jam, his mind drifts back to that earlier incident for four pages, then snaps back to the present where he's still sitting in traffic. I think I considered putting the whole flashback in italics, but decided against it.
Sometimes I'll put a flashback or dream sequence in present tense to set it off from the main text, but that didn't seem appropriate here. (Plus, sometimes editors and/or licensors object if you write a long scene in present tense.)
Glad you're liking the book, though. Dave and I really tried to wrap up most of the loose ends left dangling when the series was prematurely cancelled. As far as I know, though, this is the end. It was a four-book contract and the editor who acquired the series isn't even with the publisher anymore. I can't speak for Pocket Books, of course, but I suspect there won't be any more 4400 books. Too bad. I enjoyed writing them.
In the meantime, I'm about 36,000 words into a new CSI novel . . . thanks for asking!