I am friends with the Park Ranger for the Parkway. I will pass along this incident to him and the Park Police. A cyclist was strangled to death last year in that same area in broad daylight. The person was mentally ill and on acid as well as other drugs. He was later caugt running naked down the Parkway. Daingerfield Island is a dangerous place, day or night. Here is the link to the incident report:
http://www.nps.gov/uspp/528homdanis&arr.htm
Be careful out there. Let me know if you notice an increased police presence in this area in the next few days. I get off the Mt. Vernon trail at Four Mile Run, but occassionally ride past Daingerfield Island.