When Otto Warmbier was returned from North Korea last week after being held for more than a year, he was in a state of "unresponsive wakefulness" but otherwise stable—until he died suddenly Monday. Now NBC News reports doctors are trying to figure out what happened. One expert says Warmbier's family may have decided not to treat a medical issue—such as pneumonia, or sepsis, or a urinary tract infection—given that Warmbier was unlikely ever to recover from his vegetative state. Another says Warmbier may have suffered a pulmonary embolism, with his long flight from North Korea making a blood clot more likely. Here's what else you need to know about Warmbier and North Korea following the 22-year-old's death: