According to The Wall Street Journal, special counsel Robert Mueller has now impaneled a grand jury to investigate Russian 2016 election interference. Actually, the grand jury has reportedly been working for weeks. The White House’s lawyer, Ty Cobb, said that the White House looked forward to working with Mueller in accordance with law. But this is a major step forward for Mueller in a criminal investigation, as former federal prosecutor Andrew McCarthy points out:

This is officially criminal now. You don't impanel a grand jury for a counterintelligence investigation.
It is unclear what exactly the grand jury will be investigating; there is a second grand jury that has already been impaneled to look into the activities of former National Security Advisor Mike Flynn, which suggests that there are figures outside of Flynn who are now under the watchful eye of Mueller and his team. Grand juries are typically impaneled in order to pursue compulsion of document release or testimony, after which they may or may not initiate criminal charges.
According to CNN, Mueller’s team is focusing on precisely the sorts of material Trump said would be a red line for him: financial activities. CNN states:
Federal investigators exploring whether Donald Trump's campaign colluded with Russian spies have seized on Trump and his associates' financial ties to Russia as one of the most fertile avenues for moving their probe forward, according to people familiar with the investigation. ... the FBI is reviewing financial records related to the Trump Organization, as well as Trump, his family members, including Donald Trump Jr., and campaign associates. They've combed through the list of shell companies and buyers of Trump-branded real estate properties and scrutinized the roster of tenants at Trump Tower reaching back more than a half-dozen years. They've looked at the backgrounds of Russian business associates connected to Trump surrounding the 2013 Miss Universe pageant.
Trump attorney Jay Sekulow told CNN, “Any inquiry from the special counsel that goes beyond the mandate specified in the appointment we would object to.”
CNN says that the investigation is focusing on four key figures at least: former campaign manager Paul Manafort, former foreign policy advisor Carter Page, Flynn, and political operative Roger Stone. Manafort is apparently being targeted because investigators have found conversations among alleged Russian operatives regarding Manafort’s attempts to encourage help for the campaign from the Russians; Page, CNN reports, has been under FISA surveillance since 2014, significantly earlier than previously thought.
All of this will be fodder for speculation, but there is no evidence of collusion as of yet — and impaneling a grand jury doesn’t mean there’s enough evidence for an indictment, let alone a successful prosecution. And Trump is at least one level removed from any of the players apparently targeted by the investigation. But that won’t stop the leftist media from declaring that something corrupt happened, and blaming that corruption for Hillary Clinton’s devastating election loss.