The speculation started within days of Seth Rich being gunned down in what District of Columbia police think was an attempted robbery near his townhouse in northwest Washington.

Some on the internet wondered if Rich was killed because of his work as a staffer with the Democratic National Committee, even suggesting he had handed WikiLeaks the 20,000 emails that embarrassed the DNC and forced the ouster of its chairwoman. Others suggested he was helping the FBI expose wrongdoing in the presidential election, and that made him a target.

On Tuesday, WikiLeaks shoved those conspiracy theories into the mainstream when it announced on Twitter a $US20,000 reward for information leading to a conviction in Rich's killing on July 10. It adds to a $US25,000 reward offered by D.C. police, customary in all District homicides.

Rich's father, Joel Rich, said he was offended by what he termed "bizarre" reports that are circulating on internet discussion and message boards. Rich and his wife, Mary Ann, who live in Nebraska, where their son grew up, visited the location of the shooting last week and appealed for help in finding the killer.