Tachikawa , Japan ( CNN ) -- U.S. and Japanese authorities were investigating two small explosions Thursday outside an American air base near Tokyo , officials said . Investigators found two metal tubes and a simple timing device , which U.S. authorities described as an `` improvised launch device , '' near the Yokota Air Base . `` We can confirm there was an improvised launch device outside Yokota over the evening , '' according to a statement released by the base . `` ... We take matters of security at our installation very seriously and we are assisting the Japanese National Police in their investigation into the matter . '' No injuries were reported , and U.S. investigators combed the base for most of the night and into the early morning hours searching for possible impact points , said Capt . Ray Geoffroy , an Air Force spokesman . Residents outside the base reported hearing two small , cannon-type bangs at about 11 p.m. local time , the Tachikawa Police Department said . Daisy Gonzaga , who was staying near the base , said she heard two explosions . At first , she said she thought they were fireworks being set off as part of a Thanksgiving Day celebration inside the base . Gonzaga said when she did not see the fireworks , she thought it might be something a little more serious . CNN 's Kevin Wang contributed to this report .
Investigators found an ` ` improvised launch device '' outside of Yokota Air Base , officials say . Residents report hearing two small cannon - like explosions at about 8 am local time . No injuries or signs of violence have been reported , authorities said .