The Fire marshal usually determines the cause of the fire, be it arson or accidental, so we pretty much do all the work and the P.D. rides on our coattails. We share info with them, but we are the main investigators. If fire is arson we fill out a complaint form at the local precinct to indicate that. Any evidence we find, such as flammable liquid pour, we would take it to the P.D. lab for analysis and usually get a written report on the evidence that we submit. The Chief responding to the fire usually gives the 10-45 code, meaning a victim was found in the fire or possible removed prior to their arrival, with that 10-45 code there are 1-4 #'s describing severity of the injury. And yes a lot of paper work is involved with any fire we respond to. Usually the Chief at the fire would determine if fire is suspicious and if so we would respond.
As any old timer would attest the Fire Dept. was a necessary evil. Any thing done in the fire house we all chipped in to get it done. We even had to change the oil in the rigs by our self. As time went on things changed for the better, but I hate to say it 911 put FDNY on the map. People had a real chance to see what we do and how far we go and started to appreciate us.
One last thing, when I became a F.M. in 92 all paper work was done on a electric type writer, we then purchased "ourselves" word processors which made doing paper work so much easier. I got out in 99 and now everything is done by computer which I wasn't exposed to. So I don't really know if anything has changed from the way we did it back in the day. Maybe a newbe could chime in and let us know if much has changed.