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.