BYRON YORK: Next stop for Trump dossier probe: the FBI.

In late July, the Justice Department refused a request from the Senate Judiciary Committee — a bipartisan, joint request from Chairman Charles Grassley and Ranking Democrat Dianne Feinstein — to make two top FBI officials available for an interview in the committee’s investigation of the Trump dossier and other matters related to the Trump-Russia affair. Citing the Mueller special prosecutor investigation, Justice stated “confidentiality” and the “sensitivity of information relating to pending matters” made it impossible for the two officials, Carl Ghattas and James Rybicki, to talk to the Senate committee that oversees the FBI.

Grassley and Feinstein are still trying — they sent another, more strongly worded, request last Friday. Their efforts show the importance of the FBI in Congress’ quest to learn more about the “salacious and unverified” dossier (the words of former FBI Director James Comey), and could signal the FBI will play a key role in Congress’ dossier investigation as it plays out in coming months.

Just last week, Glenn Simpson, head of Fusion GPS, the opposition research firm that handled the Trump dossier, refused to tell Grassley’s and Feinstein’s investigators who funded the effort. But there are other ways to get at the story — and the FBI is the number-one possibility.

That’s because the FBI played a role in the case as it happened. Sometime in the process of collecting anti-Trump allegations from paid, Kremlin-linked Russian informants, Christopher Steele — the former British spy hired by Fusion to dig dirt in Russia — decided to take his information to the FBI. That appears to have been in the fall of 2016, at the height of the presidential campaign. . . .

It was a mind-boggling development: Federal law enforcement agreeing to fund an ongoing opposition research project being conducted on behalf of one of the candidates in a presidential election. In the end, the FBI reportedly did not pay Steele, possibly because of publicity concerns.

This stinks.