The mystery works began appearing in Clopper Lake in Seneca Creek State Park in Gaithersburg in May, the Washington Post reports. Each is similar but bears a different cryptic message and image. Inside the clear glass dangling from the cork by the thread is what looks like a postcard cut in the shape of a hand. It is made from two photographs stuck together. The back is affixed with antique stamps and an "air mail" sticker. A broken skeleton key is tied to the base of the hand.

The most recent find bore the message "Your question is a misunderstood answer."

State Park Ranger Sgt. Susan Hatter has been keeping the bottles in the park office as she investigates their meaning and origin. They strike her more as art than crime.

"From our point of view, there's nothing threatening about them," she said. "It drives me insane when I put my head to it. But I appreciate it. I would like to find out more, but I'm not sure we ever will."