A federal grand jury has approved the first criminal charges in special counsel Robert Mueller’s investigation of the Trump presidential campaign and its ties to Russia, a monumental step in the probe that has loomed over so much of President Donald Trump’s first year in office.

CNN first reported the news late Friday night. Reuters and the Wall Street Journal have now also reported that charges have been filed.

The indictments are sealed, so the specific charges and, most importantly, the person or persons being charged are not yet known. Arrests could be made as soon as Monday, according to CNN.

Mueller has been leading the Russia probe since May; he was appointed shortly after Trump fired FBI Director James Comey. The investigation includes exploring any possible collusion between Trump’s 2016 campaign and the Russian government, the Trump family’s financial ties to Russia, and any obstruction of justice by Trump in the investigation that was started by Comey.

Vox’s Zack Beauchamp and Andrew Prokop explained the Mueller investigation in more detail here.

What isn’t clear from Friday’s news is the target or targets of the Mueller indictments. But two prominent Trump officials have long been at the center of the Russia investigation: Former Trump campaign manager Paul Manafort and Gen. Michael Flynn, who served briefly as Trump’s national security adviser.

Manafort’s house was raided in the course of the investigation, and people close to him have been issued subpoenas. Mueller’s team reportedly told Manafort, who ran Trump’s campaign during the summer of 2016, to expect an indictment.

As Vox previously explained, “Manafort’s history of pro-Russia consulting work and experience with international skullduggery have long made him a prime suspect for potential collusion.” He is said to have attended a meeting at Trump Tower in 2016 with Donald Trump Jr. and a Russian attorney. No firm evidence of collusion has yet surfaced, but the Mueller probe has also investigated Manafort’s work before the presidential campaign, whether he properly disclosed his work for foreign entities, and whether he has ever been involved in illegal money laundering overseas.

The endgame, according to previous reports, is federal investigators want to use the pressure of criminal charges to get convince Manafort to provide more details on any possible collusion between the Trump campaign and the Russian government.

Mueller is also known to be investigating Gen. Flynn, as Vox has documented. Previous reports have suggested that Flynn may have been involved in efforts to contact Russian hackers about accessing Hillary Clinton’s emails. Flynn was also reported to have had extensive contact with Russian officials during the presidential campaign.

Flynn, whom Trump had appointed to be his national security adviser after aiding his 2016 campaign, stepped down in February once it became clear he had lied about his contacts with a Russian ambassador shortly after the election.

As of mid-Saturday morning, Trump — who suggested he could attempt to remove Mueller if the Russia investigation went in directions that he didn’t like — had not yet tweeted or otherwise commented about the pending charges.

So here is what we know for sure:

Charges have been filed and approved by the federal grand jury, the first of Mueller’s investigation

Arrests could be made in next few days

And what we don’t: