Former FBI Director James Comey this week told St. Louis Public Radio that special counsel Robert Mueller’s successful bid to get former Trump campaign chairman Paul Manafort to cooperate with his investigation shows that he may soon wrap up his probe.

In particular, Comey said that Manafort was such a big domino that it would be hard for him to imagine that Mueller would need cooperation from any other witnesses to reach whatever conclusions he finds.

“[There’s] an argument to be made that the conviction — the plea and cooperation by Paul Manafort — may represent that we’re in the fourth quarter,” Comey said. “The way you normally do investigations is you work from the bottom up, and so they’re getting pretty high.”

Comey cautioned, however, that it’s impossible to know where the investigation really is because Mueller runs it with such secrecy and doesn’t reveal what he knows until he’s ready to make it public.

“The reason I’m hesitant to even say that is Bob Mueller’s conducted his investigation like a pro,” Comey said. “You know nothing about it except through his public filings, and that’s the way it’s supposed to be. And so I can’t say with certainty where he is.”