June 30, 2013  -- Sigurder "Siggi" Thordarson considers himself the man who blew the whistle on the whistleblowers.

He is the first known WikiLeaks volunteer to have flipped on the anti-secrecy organization by agreeing to inform on it and its controversial leader, Julian Assange, for the FBI two years ago, when Thordarson was just 18 years old.

In an interview with ABC News, Thordarson claimed that by the end, he handed over "pages of chat logs, thousands of pictures, videos, images, internal documents, et cetera, et cetera" about the organization, including the text of purported chats with Assange and high-profile WikiLeaks allies.

"The FBI obtained about one terabyte of data," he said, not to mention Thordarson's personal knowledge of the WikiLeaks organization members.

While it's unclear why exactly Thordarson flipped – he said the FBI just paid him $5,000 over several months -- or what the FBI did with all that information, he is unapologetic about what former WikiLeaks spokesperson and Icelandic lawmaker Birgitta Jonsdottir called a dangerous play with people's lives.

RELATED: 'No Stopping' Release of Additional NSA Leaks, Assange Says

The FBI declined to comment for this report and some details in Thordarson's story, which was first reported by WIRED.com Thursday, as well chat logs and emails provided to ABC News by Thordarson could not be independently verified. In response to the WIRED report, WikiLeaks tweeted that Thordarson's role was exaggerated and referred to a previous statement in which the organization called him "troubled young man" who "did manage several minor tasks for the organization as one of hundreds of volunteers all over the world." The rest of the report, Wikileaks said in a tweet to ABC News, "strongly accords with our previous statements."

"In light of the relentless ongoing persecution of U.S. authorities against WikiLeaks, it is not surprising that the FBI would try to abuse this troubled young man and involve him in some manner in the attempt to prosecute WikiLeaks staff," WikiLeaks said in a statement in February, without naming Thordarson. "There is strong indication that the FBI used a combination of coercion and payments to pressure the young man to cooperate."

But to hear Thordarson tell it, it was his idea from the start. It all began in August 2011 with an email.

"I contacted the U.S. Embassy in Iceland in an email which stated that I might have information about an ongoing criminal investigation going on in the United States," the now 20-year-old Icelander told ABC News.

Thordarson said someone from the embassy called him a few hours later. When Thordarson said he had information about the WikiLeaks investigation, the embassy official said there was no investigation.

"I told them that we both knew otherwise," Thordarson said.

The same day he was invited to the embassy and met with a security official there. At the end of the meeting he was told it would take days for anyone to get back to him, if they did at all. The next day, however, Thrordarson was summed back to the embassy and this time when he met with the security official, the official suggested they go "take a walk."

"And we walked downtown, in downtown Reykjavik, and circled probably every single building there, and then we ended up in a Reykjavik hotel where I was escorted inside a conference room," Thordarson said. "Then he said his goodbyes and there were those two individuals that identified themselves as federal agents."

"They asked me first for proof of who I was so I provided them with identification… Then they asked me what I did for the organization, I explained what I did," Thordarson said.

Thordarson said he had joined WikiLeaks in February 2010 as one of many volunteers but was elevated to a key position after several staff members left the group in September of that year. He was assigned to run the organization's IRC chat room, considered a gateway into the organization for volunteers and potential sources alike.

Jonsdottir, who was a WikiLeaks volunteer herself when Thordarson joined the organization, said she tried to warn Assange about Thordarson.

"I recognized quite early on there was something not quite right about his extraordinary stories," said Jonsdottir, who is no longer affiliated with the organization.

Assange apparently did not heed her warning and Thordarson said he was given more responsibility in the organization. A little less than a year later, Thordarson said he decided to turn on WikiLeaks by contacting the U.S. Embassy.

Over the next several months, Thordarson said he exchanged emails with an FBI handler and met with agents a handful of times in Europe and once in Washington, D.C. He said he gave them with hard drives of packed with inside information, but declined when the FBI agents allegedly asked him to wear a watch with a recording device in it to tape Assange. He also volunteered to appear before the Icelandic parliament in a confidential session, which Jonsdittor attended. Thordarson provided ABC News with copies of the alleged emails and travel documents to bolster his claims.

In the emails with the purported FBI handler, Thordarson is alternatively thanked and sporadically ignored. Several times Thordarson politely asks about money he believed he was owed by the FBI. He later told ABC News the $5,000 he took from the U.S. government was not a payoff, but just meant to cover for the work he missed during their meetings.

In the interview with ABC News, Thordarson said his decision to turn on WikiLeaks came because he grew to believe Assange was "guilty of something, but crimes I'm not so sure."

Later, Thordarson clarified in an email that he didn't want to "participate in illegal hacking against government/corporations just so that WikiLeaks could have something to leak." But as WIRED reported, Thordarson was the one that once asked Lulzsec, a WikiLeaks affiliated hacktivist group, to hack the Iceland government's website.

Jonsdittor said she believes he did it because he was "terrified" of being prosecuted in the U.S. She said she's concerned about what Thordarson may have told the FBI about her former associates, worried he may have lied to the FBI as well.

"He was feeding information to the FBI about my friends and I don't know what he was saying," she said. "I believe he has some psychological issues, and that he needs to seek legal help before this media narrative spins out of control."

The U.S. government has not announced any indictment against Assange, who is living in the Ecuadorian Embassy in London to avoid extradition to Sweden to face sexual assault charges, but his attorney, Michael Ratner, told The Huffington Post earlier this month that he believes there is "more likely than not" a sealed indictment in the U.S. against the WikiLeaks founder already.

Thordarson's life as a double agent of sorts ended in July 2012 when he was booted from WikiLeaks for allegedly selling WikiLeaks merchandise online and pocketing the profits – some $50,000, according to WikiLeaks. Thordarson said it was all a misunderstanding. He said he is also facing criminal charges unrelated to his WikiLeaks work for what he called "financial crimes and tax evasion," to which he said he plans to plead guilty.

He said he chose to tell his story to the media now because he believed his role as a snitch was starting to slip out and "instead of people speculating, I decided now recently to just tell the story."

Officials at the U.S. Embassy in Iceland did not immediately respond to an off-hours call and email Saturday.