Seth Rich died on the night of July 10, 2016. The conspiracy theories were born days later. The process by which these baseless theories about his death snaked from Reddit to Twitter to Fox News illustrates just how thin the membrane has become between the conspiracy world and the mainstream.

Police say that Mr. Rich, a 27-year-old on the staff of the Democratic National Committee, most likely died in a botched robbery. But by July 13, alternate explanations emerged: A site called WhatDoesItMean.com claimed he was on his way to the F.B.I. to testify against Hillary Clinton.

Conspiracy theories help people who feel politically disempowered respond to a perceived loss of control. So it made sense, when Mrs. Clinton’s win seemed assured, that supporters of both Donald Trump and Bernie Sanders were convinced that Mr. Rich’s death showed ruthless corruption in the Democratic Party.

Conspiracy theories have been widespread among Americans for years, but they found particular prominence in the 2016 election because many people felt locked out of the process and distrustful of the news media. Mr. Trump, who repeatedly said that the press was dishonest and that the election would be “rigged” against him, fed and fed off this attitude.