Death on the run

So I met my high school classmate Todd Pepin at the start of the Freepress/Flagstar half marathon Sunday morning. I lost track of him in Windsor when I had to take a bio-break. He ended up finishing about 8 minutes ahead of me. I think that means he was lucky enough not to see a runner dying on on the road. As I turned from Bagley onto Rosa Parks an emergency vehicle went flying past. Shortly after that the race officials started squeezing the route from the full width of the road down to just a lane or two.
Then I saw why. There were several EMT types and vehicles concentrated in a tight area. As I got to within a few feet of the grouping I saw they were giving CPR to a man on the ground. The race is so large that there was still quite a grouping of us running past a full 2 hours into the race. Plus there was a relay exchange point right near this area. Everyone, from the EMTs to the runners passing by were yelling for the guy to keep fighting.
It was a very sickening feeling. I realized I've lived a very lucky life. Death has not hit too close to my family. Even as a health care profession this is the first time I'd witnessed people urgently working to save someones life.
There doesn't seem to be an easy answer to this runner's death or the 2 others. From what I've read these were relatively experienced runners in good shape. It appears to be a horrible coincidence. If I were forced to say what I think could have caused this the only thing I could even think to pin it on was the contrast in temperatures.
It was a beautiful day to run, clear and cool. It was probably in the 30s most of the morning. One spot where it wasn't so nice was the tunnel back to the US. It is hot in there, and there is a feeling that there is no fresh air in there. Then you come back out into the cold air.
That's at around the 8 mile mark so these runners still made it another 3-5 miles. Plus they've been running this route for quite a few years - it seems like there would have been more than one death in the previous races. I just don't know.
I don't feel like this will stop me from running long distances going forward. But I will certainly take the training regimens more seriously.