important to note that while these cases are associated with vaping, the exact cause is not proven. possible culprits include the vaping process itself, the substance inhaled, such as thc oil, that, of course, is the main chemical in marijuana, or some combination. tom? >> clayton sandell for us tonight. clayton, thank you. now, to the race for 2020. front-runner democrat joe biden under scrutiny tonight for a story he often tells on the campaign trail about pinning a medal on a serviceman in afghanistan. but there are questions tonight about how much of that story biden got right. here's abc's senior national correspondent terry moran. >> reporter: for years, joe biden has been recounting a story of american heroism and heartbreak on the front lines, recently telling a crowd in new hampshire how he traveled to afghanistan to pin a silver star on a young navy captain. >> one of his buddies got shot, fell down a ravine about 60 feet. this guy climbed down a ravine, carried his guy up on his back under fire and the general wanted me to pin the silver star