President Donald Trump’s legal team is preparing answers to written questions provided by special counsel Robert Mueller, according to sources familiar with the matter.

The move represents a major development after months of negotiations and signals that the Mueller investigation could be entering a final phase with regard to the President.

The questions are focused on matters related to the investigation of possible collusion between Trump associates and Russians seeking to meddle in the 2016 election, the sources said. Trump’s lawyers are preparing written responses, in part relying on documents previously provided to the special counsel, the sources said.