Japanese investigators have identified the causes of fuel leaks and other problems with Boeing's 787 but are still investigating the more serious battery problem that forced an emergency landing in January and the worldwide grounding of the jets, The Associated Press reported.

The Transport Ministry on Friday released the results of its investigation into problems that occurred with 787 Dreamliner jets in January.

It said an oil leak was caused by an improper paint job that led to a switch not working properly, while inadequate taping led to cracks in cockpit glass, and a faulty part led to braking problems.

Boeing 787 jets have been grounded for more than a month. Boeing and U.S. authorities are also investigating.

Japan Airlines and All Nippon Airways are among the biggest customers for the jets.