Live updates: Seoul Halloween crowd crush kills at least 153; Americans among 20 foreigners dead

Mehdi Moussaïd, a researcher in crowd behavior at the Max Planck Institute for Human Development, watched publicly available videos of the crush and “saw what I typically see in these kinds of accidents — there were many people, too many people in relation to the available space. [This is] measured by density, so the number of people per square meter.”

In this case, as in others he’s studied, he thinks there were about eight to 10 people per square meter.

“At that level of density, it’s no surprise that the first few people begin to faint, because they’re too tight and they can no longer breathe,” he said. “And if this continues, and that’s what happened, then all the people in that zone will no longer have enough oxygen, even after they faint, and will die one after the other.”