The investigation into possible Russian meddling in the 2016 U.S. presidential election will enter a new phase as early as Monday, when the first charges resulting from the probe could be unsealed and a target taken into custody.

A federal grand jury approved the indictment on Friday and a federal judge ordered it sealed, a source briefed on the matter has told Reuters, adding it could be unsealed as soon as Monday.

Here are the key figures who have been linked to the Russia investigation.

Donald Trump Jr