I don't think it can spread again, or at least not easily. It doesn't seem to be enough to remember simply anyone who died, you have to remember specifically the last victim immediately before yourself - somewhat analogous to It Follows. The officer who died isn't known to you the player, and on reading these files, you know enough not to try jog Alan Thomas' memories. It seems Thomas didn't know the officer particularly well, he was an outsider brought in, and Thomas can't put a name or description to him, so it doesn't seem to have kept going.