What may have been stolen is uncertain, but the cyberspies clearly took advantage of poorly protected email and scant direct notification of victims.
The hackers known as Fancy Bear, who also intruded in the US election, went after at least 87 people working on military drones, missiles, rockets, stealth fighter jets, cloud- computing platforms, or other sensitive activities, the Associated Press investigation found.
Thirty-one agreed to interviews.
Employees at both small companies and defense giants like Lockheed Martin Corp, Raytheon Co, Boeing Co, Airbus Group and General Atomics were targeted. Contacted by the AP, those companies offered no comment.
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"And if those programs are compromised in any way, then our competitive advantage and our defense is compromised."
"That's what's really scary," added Sowell, who was himself one of the hacking targets.
The AP identified Fancy Bear's prey from about 19,000 lines of the hackers' email phishing data collected by the US-based cybersecurity company Secureworks, which calls the hackers Iron Twilight. The data is partial and extends from March 2015 to May 2016.
Hackers predominantly targeted personal Gmail, with a few corporate accounts mixed in. Personal accounts can convey classified information, whether through carelessness or expediency, and lead to more valuable targets or carry embarrassing personal details that can be used for blackmail or to recruit spies.
Among their interests, the Russians seemed to be eyeing the X-37B, an American unmanned space plane that looks like a miniature shuttle.
Less than two weeks later, Fancy Bear tried to penetrate the Gmail account of a senior engineer on the X-37B project at Boeing.
The hackers also chased people who work on cloud-based services, the off-site computer networks that enable collaborators to work with data that is sometimes classified.
For example, the cyberspies tried to get into the Gmail of an employee at Mellanox Federal Systems, which helps the government with high-speed storage networks, data analysis and cloud computing. Its clients include the FBI and other intelligence agencies.
The FBI declined to give on-the-record details of its response to this Russian operation. Agency spokeswoman Jillian Stickels said the FBI does sometimes notify individual targets.
"The FBI takes ... all potential threats to public and private sector systems very seriously," she said in an email.
However, three people familiar with the matter, including a current and a former government official, previously told the AP the FBI knew the details of Fancy Bear's phishing campaign for more than a year.
Pressed about notification in that case, a senior FBI official, who was not authorized to publicly discuss the hacking operation because of its sensitivity, said the bureau was overwhelmed by the sheer number of attempted hacks.
A Pentagon spokeswoman, Heather Babb, said the department recognises the evolving cyber threat and continues to update training and technology for military, civilian and contract personnel. But she declined to comment on this hacking operation.