Westech International, a subcontractor for Northrup Grumman, said they were being cyber-attacked. 

The hackers retrieved the files via Westech International, a US Air Force maintenance and engineering subcontractor of working on the LGM-30G Minuteman III land-based intercontinental ballistic missile.

According to the news in Sky News, information is thought to have been stolen after Maze ransomware was infected into the computer network of Westech International. It is unclear if the documents stolen by the criminals include military classified information, but files which have already been leaked online suggest the hackers had access to extremely sensitive data. Due to the Maze ransomware used, US media is targeting Russia on the issue. However, it has been announced that no evidence has been reached to be associated with Russia at this time.