There are several possibilities here. One is that there are Muslims who threatened jihad violence against the beach, and who then planted glass shards and sewing needles all across it. Another is that non-Muslim hoaxers wrote the “Islamic Revolutionary Force” threat letter, and then planted the glass and needles on the beach. A third possibility is that one group, whether Muslim or non-Muslim, issued the “Islamic Revolutionary Force” letter, and another saw it and thought they would have some fun by putting the glass and needles on the beach.
In any case, this kind of thing never happened before jihad terror became a commonplace feature of life in the West.