Computers made for Portland startup Elemental Technologies contained a tiny, mysterious "stealth" chip which had been planted during production overseas to help China spy on American companies and intelligence agencies, according to a contentious, highly disputed investigation on the cover of this week's Bloomberg Businessweek magazine.

Thursday's report says Amazon discovered the spy chips in 2015, when it was evaluating the possibility of buying Elemental. (Amazon did, in fact, buy the Portland company for $296 million.)

Amazon quickly denounced the article, calling it "inaccurate" and "erroneous." The Seattle-based company, whose Amazon Web Services division hosts much of the world's online data, is highly sensitive to any suggestion of weaknesses in its technology and flatly denied having found malicious chips of the kind Bloomberg described.

Other companies, including Apple, said Bloomberg's report is fundamentally wrong.

Still, the article created an immediate sensation in the tech community and world of online security. It also left open the possibility the stealth chips could still be in use in computers Elemental sold to customers, which range from broadcast TV networks to national security clients.

If Bloomberg's report is accurate, the attack could represent an extremely serious, insidious intrusion into the core of American online security. A backdoor deep inside computer hardware could be very difficult to detect and very expensive to fix, since it might require replacing entire computers.

Amazon reported the discovery to the U.S. government, according to Bloomberg. Government investigators ultimately concluded the spy chips had been inserted into servers assembled by a San Jose, California, company, Super Micro Computer, that contracted with Chinese manufacturers to assemble technology for Elemental and other clients.

Elemental's technology enables video streaming over the internet. Its customers include major broadcasters -- ESPN, Comcast and HBO among them -- as well as some defense clients.

Bloomberg reported the spy chips were inside servers Elemental sold to its own clients, as well as computers Supermicro built for other companies. The infected chips potentially affected nearly 30 companies, according to Bloomberg, including a bank, government contractors and Apple.

"The chips had been inserted during the manufacturing process, two officials say, by operatives from a unit of the People's Liberation Army," Bloomberg reported.

"In Supermicro, China's spies appear to have found a perfect conduit for what U.S. officials now describe as the most significant supply chain attack known to have been carried out against American companies."

Bloomberg's report is based on multiple, unnamed sources within the U.S. intelligence community. It says American authorities worked under both the Obama and Trump administrations to counter the threat from the infected chips.

Nothing in Bloomberg's report suggests Elemental was aware that Supermicro had been compromised. The investigation portrays Elemental as one of many victims of the attack and says its computers were central to the intrusion's discovery.

Amazon and Apple denied aspects of Bloomberg's report, and Amazon took particular exception to many of the details in the report.

"As we shared with Bloomberg BusinessWeek multiple times over the last couple months, at no time, past or present, have we ever found any issues relating to modified hardware or malicious chips in SuperMicro motherboards in any Elemental or Amazon systems," Amazon said in a written statement Thursday. "Additionally, we have not engaged in an investigation with the government."

Amazon said a security audit conducted before acquiring Elemental found no modified chips or hardware. The company said its audit found some areas that needed improvement, but Amazon described those as "typical with most of these audits."

Apple issued its own emphatic denial Thursday.

"As Apple has repeatedly explained to Bloomberg reporters and editors over the past 12 months, there is no truth to these claims," the company wrote on its website.

"Our best guess is that they are confusing their story with a previously-reported 2016 incident in which we discovered an infected driver on a single Super Micro server in one of our labs," Apple said. "That one-time event was determined to be accidental and not a targeted attack against Apple."

Supermicro is now among the world's largest motherboard manufacturers, according to Bloomberg. It issued its own denial, writing: "Supermicro has never been contacted by any government agencies either domestic or foreign regarding the alleged claims."

Supermicro shares plunged 41 percent Thursday after the Bloomberg report.

Elemental, now AWS Elemental, continues to operate as an Amazon subsidiary and employs a few hundred people in downtown Portland. It referred inquiries to its parent company.

Bloomberg's report notes that infecting electronics hardware could be a particularly effective way for spies to build back-door access to computer systems. But it reports that authorities had thought China would be unlikely to jeopardize its role as a key global manufacturer by compromising products made there.

Three Portland engineers, led by entrepreneur Sam Blackman, founded Elemental in 2006. It became one of the most successful of a new generation of Oregon startups and helped spark a high-tech renaissance in downtown Portland. Blackman, a rising star in Portland's business community, died unexpectedly last year at age 41 of sudden cardiac arrest.

-- Mike Rogoway | twitter: @rogoway | 503-294-7699