A hound it was, an enormous coal-black hound, but not such a hound as mortal eyes have ever seen. Fire burst from its open mouth, its eyes glowed with a smouldering glare, its muzzle and hackles and dewlap were outlined in flickering flame.

—The Hound of the Baskervilles, Sir Arthur Conan Doyle

Oh, young Michael Flynn, it howls for thee. From NBC News:

The inquiry into Flynn is focused at least in part on his work with his father's lobbying firm, Flynn Intel Group, three of the officials said. It's unclear when the focus on Flynn began… Several legal experts with knowledge of the investigation have told NBC News they believe Mueller, following a classic prosecutorial playbook, is seeking to compel key players, including Flynn and Manafort, to tell what they know about any possible Trump campaign collusion with Russia. Mueller has brought onto his team a federal prosecutor known for convincing subjects to turn on associates. Any potential criminal liability for Michael G. Flynn could put added pressure on his father, these legal experts said.

What this tells us generally is that Robert Mueller and his team are getting very close to turning over some very key pieces to this puzzle.

"Any time a family member is identified as a subject that does increase pressure," said Peter Smith, a former federal prosecutor. "In the typical parent-child relationship the last thing any parent would want is for their child to get in trouble for something they initiated." That pressure appeared to mount Wednesday, when House Democrats released information they said confirmed that the elder Flynn omitted from his security clearance renewal application in 2016 that he had traveled to the Middle East in 2015 to meet with foreign leaders about a proposal to partner with Russia in a plan to build nuclear reactors in Saudi Arabia. In a letter to Flynn's former business partners who had turned over documents to Congress, Democratic Reps. Elijah Cummings of Maryland and Eliot Engel of New York also accuse Flynn of concealing the trip from background check investigators who interviewed him during that 2016 process. The congressmen, ranking members on the House Oversight and House Foreign Affairs committees respectively, told the former business partners that because "it appears that General Flynn violated federal law" they are turning over their documents to Mueller.

The elder Flynn always has looked like a promising avenue through which investigators might get to parts of the truth of what really happened between the Trump campaign and Russia. That road seems to be clearing up nicely.