I used to live (in the 80's/90's) right next to the Ballona Creek trail near Marina del Rey Middle school / S Centinela.
I remember cyclists would occasionally run past my house bleeding and mad as hell, looking for these punk teenage gangbanger types, who would hide on the trail and pull a rope across the path to trip the cyclists and then rob them.
That said, they cleaned up the trail very nicely since then but I would still be extra cautious about riding alone and near nighttime.