UPDATE 11/18/2013 2:12 PM ET: No word yet on the injured zookeeper's condition, but it's now being reported that a gazelle has become the incident's first official casualty. Spooked by the attack, the animal is believed to have run head-first into a barrier, fracturing a vertebrae. It was found dead inside its enclosure early this afternoon.
The offending zebra “needs to calm down," spokeswoman Pamela Baker-Masson, told the Washington Post. It's been separated from the animals, including the zoo's two other male Grevy's zebras -- one of whom, according to this shot grabbed by a reporter for WTOP, got close to the action:
Spokeswoman Annalisa Meyer told NBC that it's not protocol for keepers to be inside enclosures with the animals, and the zoo is investigating the incident.