Amazon's letter to consumers, sent Dec. 12, made no mention of fires. It told consumers there had been news reports of safety issues and said they could contact customer service to initiate a return if they wanted.
"With only 17 isolated, unverified complaints across thousands of hover board products and brands, we messaged customers to provide factual, informative, 'non-alarmist' guidance," the Amazon spokeswoman said. "It would have been irresponsible to incite 'alarm' or panic based on such limited data."
Helen Walter, who had bought one as a Christmas present in 2015 for her grandson in Kentucky, said as part of a court filing she didn't remember getting the letter. She did recall in the filing that soon after Dec. 12, the company that had sold her the hoverboard sent an email through Amazon reassuring her the product was safe, according to court records.
"We have sold thousands of them all over the world, we never hear any safety issue from our clients so far," a person identified as Icy wrote on behalf of the Amazon seller Cool5Pix.
More than a year later, the hoverboard went up in flames.
Cool5Pix is no longer listed as a seller on Amazon and couldn't be reached for comment.
About $41.1 million was refunded to customers by the end of January -- about 19% of all the boards sold on the site. Amazon removed 13,100 listings from 4,900 sellers, more than half from China, according to an internal company report. A few weeks later the company reinstated just 173 listings from 11 sellers that had provided additional certification documents.
Executives at the company also began working on procedures for recalling and issuing refunds to customers, along with other new safety processes, according to internal documents.
Many in the product safety world praised Amazon's actions at the time. Mr. Kaye, who was then chairman of the CPSC, said in a January written statement he wanted to "commend Amazon for voluntarily stepping up, providing a free remedy and putting customer safety first." He encouraged other retailers to stop selling boards and issue refunds.
The actions, however, didn't stop many of the boards that had come in through Amazon's platform from remaining in customer homes.
On Christmas day, Amazon's Mr. Jones said he periodically checked customer reviews: He noticed complaints of smoke, burning smells and sparks, but no fires, documents show.
In February 2016, the CPSC issued an open letter to hoverboard manufacturers and retailers telling them that any board not tested to the UL safety standards, which had just come out, was defective. That effectively meant that all hoverboards on the market were problematic. Amazon again delisted hoverboards for a period.
It took another six months before official recall notices for specific models went out through the CPSC. The commission is involved in overseeing recalls, but the notices are generally made voluntarily by manufacturers, importers or sellers. That is difficult when those entities are hard to find or outside the U.S. None of the recall notices mentioned the hoverboards listed on Amazon that had 28 fire-related safety mishaps investigated by the CPSC.
Hoverboard fire issues continued to come up at the CPSC as some of the problems they saw shifted more to other internet retailers, according to people familiar with investigations. The CPSC has reported more than 200 incidents after the time period studied by the Journal. A CPSC spokesman said the agency continues to investigate hoverboard fires.
Amazon rules now require sellers to apply in advance to sell hoverboards and submit documentation for testing to UL certification. The Journal found that most of the current listings don't contain enough information for consumers to verify that certification.
In an analysis of listings on the site in November, the Journal identified around 3,000 hoverboards for sale by 631 different sellers. Of the listings, 489 claimed to be UL certified. Just 97 -- about 20% of the listings -- disclosed basic information, such as the name of the manufacturer, needed to verify the certifications.
For years, internet giants such as Amazon, Facebook Inc., Twitter Inc. and Alphabet Inc.'s YouTube have said that they aren't responsible for what happens on their platforms, frequently citing a provision of the 1996 Communications Decency Act that says internet providers aren't responsible for what people say on their sites. Courts have often agreed.
In product-safety suits against Amazon, some have argued that its central role in sales makes it like a bricks-and-mortar store, accountable for the safety of what is on its shelves under certain state laws. Amazon has said it isn't a seller, and therefore isn't responsible under those laws.
Recent court decisions have challenged that idea. Over the summer, a federal appeals court in a case not involving hoverboards said that a customer could sue Amazon for providing an unsafe product under state liability laws. Amazon appealed and the larger court agreed to reconsider it, with a hearing scheduled for next year. Some of the hoverboard cases were on hold awaiting the outcome, but are now moving forward again.
Several other hoverboard lawsuits, including the case from Mr. Love, argued that Amazon was negligent for listing a product it knew could catch on fire. Amazon responded that Mr. Love's case hadn't presented enough evidence that Amazon knew about the fires before the sale on Nov. 22.
After one court dismissed Mr. Love's case, the Court of Appeals for the 11th Circuit said he could continue trying to make his case.
In the Fox family's case, Megan Fox purchased the hoverboard for her son from a seller named W-Deals on Amazon. The listing said little about the company or where the board was made, like a lot of the listings she and her husband had looked at.
"When we bought this, we believed we were buying it from Amazon," Mr. Fox said.
The Foxes' hoverboard arrived in a "Smart Balance Wheel" box from China in time for Christmas.
After the Jan. 9 fire, the house was uninhabitable and most of their possessions were destroyed. The family would not rebuild.
The Foxes sued W2M Trading Corp., the company Amazon said was behind W-Deals, as well as Amazon. Kevin Ma, who was named as being the registered agent for W2M in court filings, didn't respond to any of the Fox family's attempts to serve him court papers.
Five other hoverboards from W-Deals had also gone up in flames, according to CPSC investigation reports and court documents in the Fox case. An email from W-Deals in February 2016 sent through the Amazon Marketplace told consumers to "dispose of the item effective immediately" and "Use at your own risk."
The Journal couldn't reach Mr. Ma.
Later, an engineer hired by the Foxes examined their hoverboard's scorched remains and a new version of the same board, and concluded the battery likely caused the fire, a determination Amazon didn't dispute. The battery had been advertised as Samsung but was actually made by a company in China called HOPcell, the examiner found.
Reached by phone, a HOPcell representative said, "It's nonsense."
The family said in its lawsuit against Amazon that the e-retailer had violated state product liability laws by selling an unsafe product. The court disagreed, contending that Amazon didn't have enough control over the product to be considered the seller.
The family also argued that Amazon had assumed a duty to warn the Foxes about the dangers of the hoverboard. Amazon was negligent because it omitted the extent of its knowledge about the fires in its Dec. 12 letter to customers, the family argued.
That argument was also initially rejected by a federal-district court, but the Court of Appeals for the 6th Circuit in June said that the Foxes could move forward with trying to make that claim.
After the fire, the Fox family learned that the friends who recommended the W-Deals board had also seen their hoverboard catch fire. The friends hadn't thought to mention it because they didn't know it was part of a pattern, said Mr. Fox's attorney, Steve Anderson.
"They didn't think it was that big a deal," Mr. Anderson said. "They thought it was isolated."
--Shane Shifflett, Lisa Schwartz and Fanfan Wang contributed to this article.
Write to Alexandra Berzon at firstname.lastname@example.org