Tokyo (CNN) -- After eight days inspecting the Japanese company that makes the lithium ion batteries linked to problems that have led to the grounding of Boeing 787 Dreamliners around the world, aviation authorities have found no serious problems with the devices.
U.S. and Japanese officials carried out a joint inspection at the facility in the city of Kyoto where GS Yuasa makes the batteries.
While investigators found several issues in quality control at the company, they didn't uncover any serious problems that can be linked to recent incidents involving Dreamliners, Yasuo Ishii, director of air worthiness standards at the Japanese transport ministry, said Tuesday.
Two recent incidents -- a fire aboard the Japan Airlines aircraft in Boston's Logan International Airport on January 7, and a smoke alarm aboard a plane flying over Japan on January 16 -- prompted the Federal Aviation Administration (FAA) to ground all Dreamliners in the United States, and other nations quickly followed suit.
The investigation has now shifted from the battery maker to the manufacturer of the system that monitors the battery's voltage and temperature, Ishii said.
Officials from the FAA and the Japanese transport ministry on Monday began checking the quality control at Kanto Aircraft Instrument Co., in Kanagawa prefecture, according to Ishii. They haven't found any serious problems so far, he said.
Kanto Aircraft Instrument declined to comment on the matter.
The U.S. National Transportation Safety Board (NTSB) said last week it had determined that the lithium ion battery on the Boston aircraft experienced a "thermal runaway" and a "short circuit."
But safety investigators have not determined which event occurred first, or whether they were the cause or the effect of the incident.
In a thermal runaway, a battery releases energy in the form of heat, increasing the temperature of the battery and causing further damage. In an electrical short, electricity follows an unintended path.
Neither event should have occurred on the Dreamliner, Deborah Hersman, chairwoman of the NTSB, said last week.
"There are multiple systems to protect against a battery event like this," she said. "Those systems did not work as intended, we need to understand why."
The FAA allowed Boeing to use lithium ion batteries, instead of more traditional battery chemistries, under a series of "special conditions" intended to address concerns about the batteries.
Boeing has said its technical experts are working "around the clock" and are focused on "resolving the issue" and returning the 787 to service.
Currently, there are 50 wide body Dreamliners in service globally, and several hundred on order.
The 787 is highly touted because of its mostly lightweight carbon fiber construction, which airlines expect will help them save billions in fuel costs on long-haul routes.
CNN's Yoko Wakatsuki in Tokyo contributed to this report.