Special counsel Robert Mueller has asked a grand jury in Washington, DC, to investigate Russia’s meddling in the 2016 election, a major signal that the probe into whether the Trump campaign colluded with Russia is getting a whole lot bigger.

As the Wall Street Journal reports, this means Mueller’s probe has moved from simply investigating what might have happened to possibly charging people with crimes. That may not necessarily be an outcome, of course, but the fact that the investigation is widening means there is more Mueller deems worth looking into.

The call for a grand jury is done for a specific purpose: so that prosecutors can subpoena documents they might not otherwise get, ensure witnesses testify under oath, and formally charge someone with a crime if they decide to. It’s unlikely Mueller would have taken this step unless he felt there was a good reason to do so.

And there is apparently good reason. CNN reports that the special counsel has found the financial ties between Trump associates and Russia to be among the most fruitful areas of investigation.

There were signs something like this was coming. Since taking over the probe in May, Mueller has hired 16 lawyers with expertise in multiple areas including cybercrime, white-collar crime, and financial crime. Clearly, his team is looking at all angles in this investigation. The move to involve a grand jury implies there is still much more to look into over the next few months.

"This is yet a further sign that there is a long-term, large-scale series of prosecutions being contemplated and being pursued by the special counsel," Stephen Vladeck, a law professor at the University of Texas, told the Wall Street Journal. "This suggests that the investigation is bigger and wider than Flynn, perhaps substantially so."

Now, more than ever, the Trump-Russia story is not fake news. In fact, it just got very real.

Firing Mueller is an even worse idea now

There’s another problem for Trump besides the growing inquiry: It’s going to be much more difficult for him to fire Mueller.

The more serious the investigation gets, the worse it looks for Trump to ask Deputy Attorney General Rod Rosenstein to let Mueller go. Rosenstein has said he would not fire Mueller without “good cause.” If Trump wanted to relieve Mueller of his duty now, it would show he was reacting to the growing probe, as there is yet no indication of Mueller ineptly performing in his role.

Republicans have warned Trump that firing Mueller would be a bad idea. “Any effort to go after Mueller could be the beginning of the end of the Trump presidency, unless Mueller did something wrong,” Sen. Lindsey Graham (R-SC) told NBC News.

It remains to be seen how Trump will react to all of this. But the safe bet is he won’t be happy about even more people looking into his finances, his friends, and his family.