The pleas to Amazon were explicit. A food preservative sold by the online retailer and other e-commerce sites was being used as a poison to die by suicide.

“Please stop selling this product,” began one review, posted on Amazon in July 2019 by a person who wrote that a niece had used it to kill herself. “I’ve already notified Amazon and they said they would help with this but they have not.”

Since then, suicides linked to sales of the preservative through Amazon have continued. The New York Times identified 10 people who had killed themselves using the chemical compound after buying it through the site in the past two years, including a 16-year-old girl in Ohio, a pair of college freshmen in Pennsylvania and Missouri, and a 27-year-old in Texas whose mother has filed a wrongful-death suit against Amazon. Enough people purchased the preservative to attempt suicide that the company’s algorithm began suggesting other products that customers frequently bought along with it to aid in such efforts.

But when family members left behind and others alerted Amazon to the deaths and to the danger of the sales, the company declined to act.

Now, members of Congress are demanding answers. In a letter sent last week to Andy Jassy, Amazon’s president and chief executive, a bipartisan group of House members sought an accounting of the company’s sales of the preservative and related suicides, details on how the retailer had addressed the dangers, and an explanation of how it had responded to complaints.

The move comes just weeks after publication of a Times investigation that linked a website, which provides explicit instructions on suicide, to a long trail of deaths. Most were from the chemical compound, sold legally in many countries. Site members advised one another on where to buy it and how to use it. Many of those who died — The Times has now identified more than 50 people — were under 25; some were minors.

In response to the article, members of Congress have sought briefings from Google and other tech companies that help make the suicide site accessible, and have asked Attorney General Merrick B. Garland to consider ways to prosecute its operators.

In their letter to Amazon, seven House lawmakers pressed the company, saying that the ease and swiftness with which vulnerable people could buy the compound, called sodium nitrite, was a “grave concern.”

The lawmakers are targeting Amazon for questioning because they believe it to be the e-commerce site most often used to buy the compound and get it quickly delivered, and because of claims by parents and others that product reviews on Amazon warning about the danger were removed, said Representative Lori Trahan, Democrat of Massachusetts and a member of the House Energy and Commerce Committee.

In a written response to the lawmakers on Thursday, Brian Huseman, Amazon’s vice president for public policy, extended condolences to families of the dead while defending Amazon’s practices and sales of the compound. He said it was used for a range of purposes and was available from other retailers.

“Amazon makes a wide selection of products available to our customers because we trust that they will use those products as intended by the manufacturers,” he wrote. “Like many widely-available consumer products,” he added, the compound “can unfortunately be misused.”

The lawmakers found the company’s answers insufficient.

“Amazon had the opportunity with their response to collaborate with us on this issue that’s tragically ending the lives of people across our nation,” Representative Trahan said. “Instead, they failed to answer many of our most critical questions”

In email exchanges with The Times, an Amazon spokeswoman declined to comment on the 10 deaths that The Times identified.

Other sites said they had restricted sales of the compound.

Last year, an eBay director wrote to a coroner in England that the company had prohibited global sales of the compound in 2019 after receiving a report of its potential use in suicides. However, The Times identified eight suicides involving eBay sales of the poison since then, including a death the coroner was reviewing.

EBay did not respond to detailed emails and messages seeking comment. But in the letter to the coroner, the eBay director acknowledged that despite the ban, it was possible for “unscrupulous or unaware sellers to circumvent our policies and filters.” He wrote that the company would support government restrictions on online sales of the chemical to prevent future suicides.

In November 2020, Etsy banned sales of the compound, said a spokesperson, who declined to explain why. An Etsy customer posted in May 2018 that he was planning to use his purchase to kill himself. In August 2020, a 35-year-old in Mississippi wrote on the suicide site that he had bought the compound on the site. Days later, he was dead.

The United States is among many countries that allow the chemical compound to be sold as a food preservative, and the federal Food and Drug Administration regulates its use for that purpose.

There is no systematic tracking of suicides involving the compound, but The Times identified dozens of people who had used it since 2018 in the United States, the United Kingdom, Italy, Canada and Australia. More than 300 members of the suicide website had announced intentions to use the compound to kill themselves.

A study of 47 cases of poisoning by the preservative reported to the National Poison Data System over a five-year period found that suicide attempts with it had been increasing since 2017. A 2020 article in the Journal of Emergency Medicine warned that because the compound “is readily accessible through online vendors, and is being circulated through various suicide forums,” emergency rooms might see more patients who have used it.

Dr. Kyle Pires, a resident emergency room physician at Yale University Hospital who treated a 28-year-old woman who had bought the compound on Amazon, wrote in the journal Clinical Toxicology about her death and the recent rise in suicides by this method.The article, published last May, said policymakers should be aware of the preservative’s use in suicides, and encouraged emergency rooms to stock doses of an antidote, methylene blue, that can prevent death if administered early.

In an interview, Dr. Pires said that businesses should be able to buy the preservative, but sales to individuals should be banned.

“There’s an argument that it’s a slippery slope to restrict sales of something that is legal just because some people are using it to kill themselves,” Dr. Pires said. “This is a cost-benefit analysis of a small number of hobbyists using this chemical to cure meat at home versus these growing numbers of young people, including teenagers, using it to kill themselves. For me, it’s an easy calculation.”

In the United Kingdom, coroners for nearly two years have been highlighting suicides involving online purchases of the preservative and asking the government to take action. A cross-government group is working with businesses — including manufacturers and online suppliers of the preservative — to reduce access and end some sales to individuals, according to a spokeswoman for the government’s Department of Health and Social Care. The United Kingdom already requires sellers to inform law enforcement officials of any suspicious purchases of the compound, though it’s unclear how often such reports are made.

Some businesses have gone further.

Metalchem, a British vendor, stopped selling the compound to the public in April 2020 after learning that it had been used for suicide. Mike Keay, the company’s chief executive, also notified an English coroner that he had asked other businesses to stop selling the compound online “when the reason for the purchase cannot be reasonably ascertained.”

“Sadly, nearly two years later and the preservative is still available online, even on Amazon, with worldwide shipping,” Mr. Keay wrote in an email to The Times this week.

In the United States, Amazon continued to receive complaints about its sales of the compound — including, in May 2020, from someone whose father had just used it to die; in October 2020, from the grieving mother of an 18-year-old who had killed himself; and last year from Ruth Scott of Schertz, Texas, who is now suing the company.

Her 27-year-old son, Mikael, who had struggled with depression, learned about the compound on the suicide website and bought it on Amazon. He killed himself in December 2020.

Ms. Scott said she had reached out five times to inform Amazon, only to hit brick walls. A customer service representative wrote to her that her message would be passed along.

“I am sorry for your loss,” said the email, which was reviewed by The Times. “But at least your son is now on our God’s hand.”

After Carrie Goldberg, a lawyer for Ms. Scott, wrote to Amazon’s general counsel and implored the company to remove the product from its platform, lawyers for Amazon pointed out a Texas law and court decisions protecting the seller of a legal product used in a suicide.

“They know it’s killing people,” Ms. Scott said in an interview. “They are fully aware. They just don’t care.”