I read about it in Jim Hougan's notes on Jim Jones. The source he references are English-language Brazilian news articles from 1978, which are not easy to find:

• "To Brazilians, Jim Jones was a CIA Agent" by O Globo
• Brazil Herald, "The little-known story: Jim Jones' early days in Rio de Janeiro" by Harold Emert

Jim Hougan is a solid private investigator. He has the best account of the Watergate scandal, and covered some pretty big union struggles in the 90's. I doubt he would just make up sources for no gain. The only question is the veracity of the sources.

The article is an interesting take on the Jonestown massacre. Basically, US Gladio:

What follows is a work in progress about Jim Jones and the Peoples Temple. In so far as it has a central thesis, it is that Jones initiated the 1978 massacre at Jonestown, Guyana because he feared that Congressman Leo Ryan's investigation would disgrace him. Specifically, Jones was afraid that Ryan and the press would uncover evidence that the leftist founder of the Peoples Temple was for many years an asset of the FBI and the CIA. This fear was, I believe, mirrored in various precincts of the U.S. intelligence community, which worried that Ryan's investigation would embarrass the CIA by linking Jones to some of the Agency's most volatile programs---including "mind-control studies" and operations such as MK-ULTRA.

This is, I believe, why Jones's 201-file was purged by the CIA immediately after Jones's case-officer, Dan Mitrione, was murdered in Montevideo, Uruguay. [1] What I believe and what I can prove are, in some instances, two different things. There is no smoking gun in the pages that follow. But I think the reader will agree that there are certainly a great many empty cartridges lying around---enough, perhaps, to stimulate further investigation by others.

Having said that, it should be added that I am hardly the first to suggest that the Jonestown massacre was the outcome of someone's secret machinations. The affair is inherently mysterious, and conspiracy theories abound---the most prominent among them that "Jonestown" was a CIA mind-control experiment.

This is a view that has been put forward in a number of venues. Congressman Ryan's close friend and chief-of-staff, Joe Holsinger, is persuaded of it. The respectable Edwin Mellen Press has gone so far as to publish a book on the subject.[2] And professional conspiracists such as John Judge have embraced the thesis wholeheartedly.

In my view, they're probably mistaken. The truth is darker, the evil more banal.