(CNN) 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.

"We are in continuing discussions with the special counsel and we do not comment on those discussions," said Trump attorney Jay Sekulow.

There may be more rounds of questions after the first answers are returned. The special counsel had insisted that there be a chance for follow-up questions as well. But after a prolonged back-and-forth over months, the two sides agreed to start with a first round of questions.