Three parents whose children were killed at Sandy Hook Elementary School in 2012 filed a defamation lawsuit on Tuesday against Alex Jones, the right-wing conspiracy theorist who has long claimed the shooting was “completely fake” and a “giant hoax” perpetrated by opponents of the Second Amendment.

Mr. Jones, the popular radio show host who also operates the conspiracy theory website Infowars, has questioned for years whether 20 children and six adults died in the school massacre in Newtown, Conn. To bolster his false claims, he often cites news reports and video clips from the hours after the shooting that turned out to be incomplete or based on wrong information.

Soon after they buried their children, many Sandy Hook parents started to come under fierce attack by conspiracy theorists who have said they are actors in an elaborate scheme to enact stricter gun control laws. The fringe theories still thrive in small forums online but have reached a far greater audience through Mr. Jones, the most vocal propagator.

The two lawsuits filed on Tuesday represent the first civil action taken by parents accusing Mr. Jones of defamation. One was filed by Leonard Pozner and his former wife, Veronique De La Rosa, and the other was filed by Neil Heslin. Their sons, Noah Pozner and Jesse Heslin, both 6, were killed at Sandy Hook.