House Intelligence Committee chairman Devin Nunes has issued a subpoena to David Kramer, a former State Department official who, in late November 2016, traveled to London to receive a briefing and a copy of the Trump dossier from its author, former British spy Christopher Steele. Kramer then returned to the U.S. to give the document to Sen. John McCain.

Kramer is a senior fellow at the McCain Institute for International Leadership at Arizona State University.

McCain later took a copy of the dossier to the FBI's then-director, James Comey. But the FBI already had the document; Steele himself gave the dossier to the bureau in installments, reportedly beginning in early July 2016.

McCain, recovering in Arizona from treatments for cancer, has long refused to detail his actions regarding the dossier. For his part, Kramer was interviewed by the House Intelligence Committee on Dec. 19. The new subpoena stems from statements Kramer made in that interview.

In the session, Kramer told House investigators that he knew the identities of the Russian sources for the allegations in Steele's dossier. But when investigators pressed Kramer to reveal those names, he declined to do so.

Now, he is under subpoena. The subpoena, issued Wednesday afternoon, directs Kramer to appear again before House investigators on Jan. 11.

Knowing Steele's sources is a critical part of the congressional dossier investigation, for both sides. If one argues the document is unverified and never will be, it is critical to learn the identity of the sources to support that conclusion. If one argues the document is the whole truth, or largely true, knowing sources is equally critical.

Beyond that, there is another reason to know Steele's sources, and that is to learn not just the origin of the dossier but its place in the larger Trump-Russia affair. There is a growing belief among some congressional investigators that the Russians who provided information to Steele were using Steele to disrupt the American election as much as the Russians who distributed hacked Democratic Party emails. In some investigators' views, they are the two sides of the Trump-Russia project, both aimed at sowing chaos and discord in the American political system.

Investigators who favor this theory ask a sensible question: Is it likely that all the Russians involved in the attempt to influence the 2016 election were lying, scheming, Kremlin-linked, Putin-backed enemies of America – except the Russians who talked to Christopher Steele?

On the other hand, the theory is still just a theory, for now. And that is one reason House investigators seek Steele's sources – and why they will try to compel Kramer to talk.