In early 2015, an anonymous comment accusing a Florida company of being a scam was posted on a consumer website called RipoffReport.com.

Around that time, the publication’s phone rang. The caller said he was Matthew Whitaker—now the acting attorney general—and he was angry, said Ed Magedson, owner of Ripoff Report. Using profanity, Mr. Whitaker demanded the removal of all negative reports about the company, World Patent Marketing Inc., Mr. Magedson said.

“He threatened me using a lot of foul language,” said Mr. Magedson, who added he had reviewed notes he made at the time. “He threatened to ruin my business if I didn’t remove the reports. He [said he] would have the government shut me down under some homeland security law.”

Ripoff Report itself has been subject to criticism, with detractors saying it has profited from the negative reports posted on its website.

Since President Trump appointed Mr. Whitaker last week, the acting attorney general has faced questions about the extent of his involvement with World Patent Marketing, where he was a paid advisory-board member until at least 2016. The company was shut down last year by the Federal Trade Commission after it accused the firm of scamming $26 million.

The threatening phone call recalled by Mr. Magedson suggests Mr. Whitaker took a more active role than previously known in shielding World Patent Marketing from outside criticism. He also wrote an August 2015 email threatening an unhappy customer, court documents show.

The call also suggests Mr. Whitaker was aware of allegations of fraudulent activity against the company, which was later confirmed by the FTC in its accusations.

On Wednesday, top Democrats in the House of Representatives, who will take control of the chamber in January, sent letters to Mr. Whitaker, the FTC, the founder of World Patent Marketing and others requesting more information about Mr. Whitaker’s role.

A Justice Department spokeswoman has said: “Acting attorney general Matt Whitaker has said he is not aware of any fraudulent activity. Any stories suggesting otherwise are false.”

On Wednesday, the Justice Department declined to offer a new comment and referred to the previous statement.

World Patent Marketing, which started in early 2014 and was run from Miami Beach, Fla., offered to patent and promote ideas from would-be inventors. The FTC said the company delivered little and threatened customers who complained.

Ripoff Report, of Tempe, Ariz., lets people post anonymous complaints about companies on its website. It has been frequently sued and accused of profiting from the negative reports by offering a paid service to clean up the reputational damage. Mr. Magedson said his firm tries to verify some comments and helps companies deliver better customer service.

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