Lewis & Short

investīgātĭo, ōnis, f. [investigo], a searching or inquiring into, investigation (class.): rerum occultissimarum, Cic. Fin. 5, 4, 10: veri (with inquisitio), id. Off. 1, 4, 13: sapientiae ejus, Vulg. Isa. 40, 28.