Officials at the Franklin Park Zoo are still unsure how a pair of Grevy’s zebras escaped from their exhibit over the weekend, shutting down the park until they were located. The zebra foal escaped Saturday, her first day in the exhibit, by breaking through an electric fence. Her mother went looking for her and also escaped. The system had been tested and was functioning before the zebras were put into the exhibit, officials said. The zebras will not be returned to the exhibit until steps have been taken to prevent another escape, Zoo New England said.