Questions of collusion between the Trump campaign and Russian officials arose beginning in early 2016. The alleged first contact between the Trump camp and Russia was made by George Papadopoulos, a young campaign adviser who met with a London-based professor offering a connection with Russian officials. Papadopoulos suggested he could coordinate a meeting between Trump and top Russian officials several times, and was arrested in July by FBI agents investigating the case. He pleaded guilty in October of this year.

In July 2016, President Trump’s son, Donald Trump Jr., arranged a meeting between a Russian lawyer and Trump staff Jared Kushner and Paul Manafort at the recommendation of a publicist who has ties to both Russia and Trump. Those involved in the meeting maintain that they primarily discussed adoption policies, although many are skeptical of the real basis for the gathering. This meeting and what was discussed there is at the root of the ongoing investigation.

The investigation into the possibility of collusion between Trump and Russia is headed by special counsel Robert Mueller. In July 2016, FBI director James Comey came on board, granting Mueller access to all past FBI findings in connection to the Russia scandal. In May, Trump fired Comey, citing his handling of presidential candidate Hillary Clinton’s use of personal emails. In a dramatic testimony to Congress, however, Comey said that Trump had demanded a statement of loyalty from him, which he refused to give, possibly prompting his sacking.

Also removed from the investigation was Attorney General Jeff Sessions, who recused himself in March. During his first testimony to Congress in October, Sessions claimed to forget a meeting with Russian Ambassador Sergey Kislyak and lied about having communicated with Russian officials during the campaign.

Mueller’s investigation also led to the prosecution of former national security adviser Michael Flynn. In December, Flynn pleaded guilty to making “false, fictitious and fraudulent statements” to the FBI about the nature of his private meeting with Ambassador Kislyak. He served a short tenure as part of the Trump cabinet and was removed after it was revealed that he had misled Vice President Mike Pence about the same meeting.

As Mueller continues to probe into this increasingly complex situation, more actors and events may emerge to explain the remaining mysteries of the Trump-Russia saga. 2017 was marked with notable developments, and it’s likely that more prosecutions and discoveries will continue to plague the Trump administration moving into 2018.

For a full timeline of events, check out Vox’s visual guide and BBC’s description of the main players.