Thinking out loud here with the idea I may be able to enlighten everyone about the relationship between Long Island (the main island) and Fire Island.
I grew up on Long Island and spent quite a few years traveling from the main island to Fire Island during the summer. Memorial Day kicks off the summer season when ferryboats are available to travel from the main island to Fire Island. The ferries run twice a day. Once in the morning to Fire Island and back, and again to Fire Island and back. For the most part, those ferries carry visitors who want to enjoy the municipal beaches of Atlantique or Ocean Beach. Doing so is a summer ritual going back long before I was a teenager (1967).
Although the beach runs the length of Fire Island, not all of the beaches are open to the public. Some are directly behind private residences and are not accessible from the road that runs the length of the island. All of them are accessible if you are willing to walk from the municipal beaches along the shoreline to other municipal beaches. Between those beaches are the beach areas used by private residents in the area.
Oftentimes, we would arrive at Atlantique Beach early in the morning, then walk via the beach route along the water to Ocean Beach. It was a long walk, but it afforded us an opportunity to glimpse another way of life and perhaps see a celebrity or two. Ocean Beach hosted an annual summer drag ball as far back as I can remember. Oftentimes, we saw young men taking the ferries from the main island to Fire Island carrying beautiful, sequined evening dresses on hangers covered with dry cleaning bags, and wig boxes. Over time, Ocean Beach came to be associated with the gay community, although it was also the home of members of New York City's more affluent, theater community, writers, and professional people. Ocean Beach was a kind of offbeat place for wealthy people to go slumming in relative peace.
Fire Island, where the designated, municipal beaches are, can be traversed by a series of boardwalks from the main island side, over Fire Island itself to the ocean side. In some places, it can be easily walked in about 10 minutes. There are also areas where there are thickets of swamp reeds, sea grapes, and other bramble-type bushes and shrubs. They are so thick and the brambles and stickers so nasty, that you would have to be an idiot or a very determined soul to try to gain access to the ocean side beach through them. I cannot imagine someone fighting their way through those brambles with heavy body parts, simply to stash them on the beach. The best route is to do it either from behind a private home, where the thickets have been chopped down to allow access to the beach, or along the routes cleared for the municipal beaches (which is highly unlikely as they're quite wide open and sometimes lighted). So, my guess is the killer had to access the beach through the back yard of a private residence.
Burlap. As I mentioned, the swamp thickets grow naturally along the beach and provide a windbreak for the wind blowing from the ocean over the island. It gets very windy by the ocean's edge. During certain months of the year, the wind is so strong that it blows sand from the beach into the back yards, homes and the road that stretches the length of the island. As a result, wherever those swamp thickets have been cleared for beach access or private homes, municipal workers and landscapers plant what's called "balled and burlapped" conifers along the dunes to prevent wind and sand erosion. A "balled and burlapped" conifer is a salt-resistant, small stature tree of about 6' tall with its roots wrapped in burlap and held together with wire or hempen cord. In order to plant this small tree, you have to dig a hole, cut the wire or cord, unwrap the burlap from the roots, put the tree in the ground and then water it. On a property with a 100' property line close to the ocean, you can easily place 30 trees in a line to form a wind break. That's 30 large pieces of burlap. That's a lot of burlap.
Now there's a number of things you can do with that burlap. Ordinarily, if you're an independent landscaper working for a wealthy person, you gather the burlap, chuck it back in your truck and take it home or back to your nursery. You don't leave a pile of burlap on a wealthy customers back lawn. You take it away with you. The same goes for municipal workers. After you plant, you clean up. You bring the burlap back to your municipal work station. Once the burlap is back on your own turf, you can either save it for future use, burn it, or dispose of it. Most landscapers hold some of it back for future use, but don't stockpile it, because if a pile of burlap remains in the sun for too long...it spontaneously combusts. Yes, it catches on fire, just as grass clippings do. Nurseries, on the other hand, will keep a lot of it around for future use with a directive to have the employees "turn" it (flip it over) so that it doesn't catch on fire. They will also keep some for customers when they ask for small pieces for home gardening.
How do I know all of this? My father was a landscaper on Long Island from 1950 until 1971.
When I read about these murders, I immediately thought about that burlap and who might have access to it. My conclusion was that it had to be a landscaper, a nurseryman, a municipal worker, a carpenter working where new home construction required new installations of "balled and burlapped" trees and plants, or a nursery worker. Outside of that, I cannot think of another profession or reason for someone to require a constant or secure knowledge of where burlap may be used, stored or obtained.
Hope this has been helpful.
That was definitely enlightening! I wonder though if the wealthy person might request for them to use the burlap saying they have a project for it or whatever. I can't imagine it'd be such an outlandish request that someone Years later would remember it when the GB4 were discovered. Or maybe they did and called it in, who knows.