A local vibrational mode occurring at 638 cm-1 in nitrogen-rich GaAs bulk crystals and 14N-implanted GaAs layers has been investigated by high-resolution Fourier transform infrared absorption spectroscopy. Measurements on samples coimplanted with 14N and 15N show that the core of the defect consists of a single nitrogen atom. In bulk crystals, a pronounced four-line host-isotope fine structure appears, due to bonding with three nearest-neighbor gallium atoms. A valence-force model is presented which reproduces quantitatively the experimentally observed line shape. The comparatively high vibrational frequency is attributed to a shortening of the nitrogen-gallium bond length with respect to the isolated substitutional nitrogen atom. The results are consistent with an assignment to a nitrogen-gallium vacancy complex.