It started as a joke and ended up as a shot heard round the Internet, with the joker losing his job and Wikipedia, the online encyclopedia, suffering a blow to its credibility.

A man in Nashville has admitted that, in trying to shock a colleague with a joke, he put false information into a Wikipedia entry about John Seigenthaler Sr., a former editor of The Tennessean in Nashville.

Brian Chase, 38, who until Friday was an operations manager at a small delivery company, told Mr. Seigenthaler on Friday that he had written the material suggesting that Mr. Seigenthaler had been involved in the assassinations of John and Robert Kennedy. Wikipedia, a nonprofit venture that is the world's biggest encyclopedia, is written and edited by thousands of volunteers.