Questions remain over the disappearance of FBI informant in Russia

The detentions of U.S. citizens, like reporter Evan Gershkovich, Brittney Griner and Paul Whelan by Russia, has kept tensions between the U.S. and Russia high as the war in Ukraine rages on.

Wall Street Journal national security reporter Brett Forrest has taken a look at another case, the 2015 disappearance and murder of Billy Reilly, an FBI informant in his new book, "Lost Son: An American Family Trapped Inside the FBI Secret Wars."

Forrest spoke about the case and his book with ABC News Live Monday. The FBI didn't immediately comment on the book and Forrest's reporting.

ABC NEWS LIVE: So your book dives into the post 9/11 world and the FBI's Confidential Human Source program. Explain what that program is and how Billy Reilly got pulled into it, and the world of global intelligence.

BRETT FORREST: Well, the FBI, since its foundation, has used cooperators and informants as a fundamental part of its work. But after 9/11, when Congress and the administration mandated that the FBI do more to be proactive in its prevention of terrorist conspiracy, the bureau really stepped up its game with such people and reconfigured their approach with them.

They created something called the Confidential Human Source program, which ultimately they used to gather not just evidence to be used in courtrooms, but intelligence to be used outside of DOJ.

ABC NEWS LIVE: And your book is based on an article that you wrote back in 2019 called "The FBI Lost Our Son," where you followed Billy's tracks up until the point where he was found without revealing what happened to him. Can you explain how he went missing?

FORREST: Billy Reilly was a young man growing up outside Detroit, and he came of age after 9/11 and was fascinated by global conflict and world religions and foreign languages. And that brought him to the attention of the FBI, as internet traffic did ultimately after the war in Ukraine broke out in 2014.

A year after that, Billy traveled to Russia. His parents weren't sure why he went and he disappeared there.

ABC NEWS LIVE: How do you hope or perhaps expect people to react at home when hearing his full story?

FORREST: Well, I think fundamentally the lesson we have here is that the FBI and other federal law enforcement and intelligence agencies continue to demand our scrutiny, and they need oversight.

In the case of Billy Reilly, the FBI has not been forthcoming with the family, nor with myself and others who are trying to get answers, including folks on Capitol Hill. And his story, the ending, at least, and the FBI's involvement in it remains a mystery.

ABC NEWS LIVE: Your colleague, Wall Street Journal reporter Evan Gershkovich, is still being held. From your research, what can families like his hope for when it comes to dealing with Russia?

FORREST: Well, not every American who goes missing or is detained in Russia gets the same treatment from the U.S. government. Evan, despite the negative experience that he's undergoing, at least has the support of the U.S. government who has designated him as wrongfully detained.

There are other people, other Americans in Russian prisons who have been there for years under questionable circumstances, who have been forgotten. Nonetheless, all these people face a terrible fate of not really being able to affect their circumstances.

ABC NEWS LIVE: You've also been on the ground in Ukraine covering the many brutal battles of the Ukraine-Russia war more than a year into this conflict. What's your biggest takeaway from your time in the field there?

FORREST: Well, I have had quite a number of years in both countries, so for me, it's been a terrible personal experience as well. And we all just want to figure out how this could possibly end. And that remains a big question mark because both sides have told themselves that they're winning, at least to some degree. And that makes it very difficult for them to come to the negotiating table.

ABC NEWS LIVE: Would you go to Russia at this point for work or personal reasons or otherwise?

FORREST: I think that's probably not a good idea, especially given what's been happening with Evan. This is something I think many of us were afraid of as a possibility once the war began and the worst forces in Russia came to the fore.

The genie is out of the bottle in Russia. And I don't think it's necessarily a good idea for folks to go over there and work there at least Americans.