Amazon is being told to reveal how it decides which products get the "Amazon's Choice" label in its online store.
Two US senators have written to Amazon asking it to say whether people or algorithms are making decisions about what gets the label.
They are worried that the Choice category can be manipulated via fake reviews and can mislead customers.
Amazon said it "worked hard" to ensure reviews were accurate and real.
The online retail giant has been given until 16 September to respond to the letter.
The letter was written following an investigation by news site Buzzfeed which claimed many products in the "Choice" category are of poor quality or have their ratings boosted by fake reviews.
Research suggests products getting the Choice label sell better. OC&C Strategy Consultants found that products awarded the Choice label see a sales jump of about 300%.
This is partly because anyone using their Amazon Echo smart speaker to buy products in a category in which they have never shopped before, will get a product bearing the Choice label.
"We are concerned the badge is assigned in an arbitrary manner, or worse, based on fraudulent product reviews," wrote Democrats Bob Menendez and Richard Blumenthal.
A lack of transparency about Amazon Choice means consumers were prevented from making "informed choices", they said.
The senators want answers about the process behind Choice and whether the products in the category are ever checked by people to ensure they are not faulty or badly made.
Mr Menendez told Buzzfeed he wanted a "full and robust" answer to the queries in the letter. This could prompt a further call to Amazon to change the way Choice products are picked, he said.
"Or we force it to change by some federal regulatory agency or by some legislation," he added.
In a statement, an Amazon spokesperson said: "Amazon invests significant resources to protect the integrity of reviews in our store because we know customers value the insights and experiences shared by fellow shoppers."
It said it used both human investigators and automatic tools to find and remove fake reviews. It said it policed its reviews, worked with social media sites to stop fake reviews being generated and took legal action against offenders.
"We work hard to enrich the shopping experience for our customers and selling partners with authentic reviews written by real customers," it said. "Customers can help by reporting any requests they get to manipulate reviews to customer service."