The Wall Street Journal said the non-profit Free and Fair Markets Initiative had cited support from a union, a management professor in Boston and a businessman in California.
But the newspaper found that the financial backers of FFMI included shopping centre owner Simon Property Group, well-known retailer Walmart and software firm Oracle.
Simon Property is battling to keep customers who had been drifting over to Amazon, Walmart is a retail competitor to Amazon, while Oracle is competing against Amazon to win a US$10 billion defence contract.
The Boston academic said the FFMI had ghost-written an op-ed about Amazon in his name but he was unaware that he had been named as a member of the group.
And the businessman in question had been dead for months before his name was take off the FFMI website.
The WSJ said FFMI had refused to say from where it was getting its funds or to identify its office-bearers.
“The bottom line is that FFMI is focusing on the substantive issues and putting a spotlight on the way companies like Amazon undermine the public good — something that media outlets, activists, and politicians in both parties are also doing with increasing frequency,” the organisation told the newspaper in a statement.
“If Amazon cannot take the heat, then it should stay out of the kitchen.”
FFMI is run by communications outfit Marathon Strategies which was founded by political operative Phil Singer. He has been an aide to US Senator Chuck Schumer and worked on Hillary Clinton's 2008 bid for the Democrat presidential nomination.