Some authors have theorized that some of the basic letters of hangul may have been influenced by the 'Phags-pa script of the Mongol Empire, itself a derivative of the Tibetan alphabet, a Brahmi script (see origin of Hangul).[12][13] However, one of the authors, Gari Ledyard, on whose work much of this theorized connection rests, cautions against giving 'Phags-pa much credit in the development of Hangul: