Hackers broke into US Department of Defense computers and downloaded terabytes of data containing design information about the Joint Strike Fighter, a $300bn stealth fighter currently under development, according to The Wall Street Journal.
The stolen files all relate to the design of the Joint Strike Fighter and its electronic systems, the Wall Street Journal reported, saying they could be used to help defend against the jet.
However, the most sensitive files were not compromised since they are stored on computers that are not connected to the internet, the report said.
The reported attack raised more questions than it answered.
For example, the report did not say how attackers managed to download terabytes of data before being discovered. A single terabyte can take up to several weeks to download over a relatively fast data connection, such as a DSL or cable modem.
The report also suggested China could be behind the stolen data, noting that investigators believe the attack originated in China. However, it said the exact identities of the people behind the attack had not been established.