In a statement to ABC News, which first reported on the lawsuit, Fox News commentator Ed Butowsky-a defendant in the suit-called it "one of the dumbest" he's seen, adding "to file a lawsuit to say that anybody has benefited just smells weird".
US intelligence officials have since debunked that theory, stating that the emails were in fact stolen by Russian state-sponsored hackers in an attempt to "interfere with the USA election process".
Rich was killed in his D.C. neighborhood in July 2016 in what local police said was a botched robbery attempt, The Washington Post reported.
"Joel and Mary Rich have brought this case to hold Fox News, Fox reporter Zimmerman, and political operative and Fox News contributor Butowsky, accountable for their reprehensible actions", Leonard A. Gail, a lawyer representing the family, said in a statement. The news outlet said it "not initially subjected to the high degree of editorial scrutiny we require for all our reporting".More news: Halep, Venus advance, Wozniacki knocked-out
More news: CL: Barcelona happy with a goalless draw against Chelsea, says Sergio Busquets
More news: Netflix pays its queen less than her consort on 'The Crown'
Wikileaks itself fuelled the conspiracy theory by offering a reward for the capture of Mr Rich's killer and hinting that he may have been the source of the emails.
In August, a private detective hired by the Rich family sued Fox News for defamation and racial discrimination, claiming that the network had fabricated quotes attributed to him as part of a coordinated effort with the White House to smear Rich. (Fox News denied the charges.) The detective, Rod Wheeler, who was himself a Fox News contributor, also claimed that Butowsky had coordinated with the Trump administration to plant the story. It also claims that Zimmerman provided similar information to Joel Rich, which the Riches allege was false.
Fox News's May 2017 story, which it later retracted, cited unidentified law-enforcement sources for such a claim. Both Zimmerman and Butowsky's "sources", as well as the Federal Bureau of Investigation and local police, immediately disputed the account - and Rich's family issued a statement requesting that Fox News retract the story because it was based on "untruthful rumors".
"Joel and Mary have been irreparably harmed because whatever opportunity that existed to investigate and find closure with regard to their son's murder was taken away from them", the lawsuit continues.
Since he didn't write the article, suing him over it amounted to a "frivolous, baseless lawsuit". The false claim, they said, led to their son's murder being treated as a "political football", and are suing the network, and Zimmerman and Butowsky, for their pain and suffering they say they've endured.