See the little blue arrows in that video? Those are storm chasers in their vehicles. Now see that big white blob that moves across the screen? That's the tornado in El Reno, Oklahoma that is considered the widest one in U.S. history and killed four storm chasers on Friday.
It's not clear from this Spotter Network graphic which triangle represents the three TWISTEX storm chasers who died during the event. But it does make clear just how many of them were chasing this monstrous EF5 tornado that killed a total of 18 people last week.