WASHINGTON — President Donald Trump's lawyers are preparing answers to questions submitted by special counsel Robert Mueller, a source familiar with the matter told NBC News on Thursday.

The questions are focused on the issue of whether the Trump campaign colluded with Russia during the 2016 presidential race, the source said. However, the source stressed that these questions are a refined version of questions that have gone back and forth between the two sides for months.

CNN was first to report that the president's legal team was preparing answers to the written questions submitted by Mueller.

Mueller is investigating Russian interference in the 2016 U.S. presidential election, which includes probing whether there was any collusion between Trump campaign associates and the Kremlin. The special counsel's office is also investigating whether there was any obstruction of justice.

In his public remarks, Trump has repeatedly denied any collusion took place.

In April, The New York Times reported that Mueller had provided dozens of questions to Trump 's lawyers, which included inquiries about Trump's ties to Russia, his firing of FBI Director James Comey and his businesses, among other topics.

There is still no agreement on an in-person interview between the president and Mueller's team, the source familiar with the Trump legal team's efforts said Thursday. In the past, the president has publicly expressed a willingness to talk to Mueller, even under oath.

Rudy Giuliani, another member of Trump's legal team, did not respond to requests for comment.

The office of the special counsel did not respond to a request for comment.

Kristen Welker reported from Washington, and Dartunorro Clark reported from New York.