Washington( CNN) Lawyers working with a team led by special attorney Robert Mueller approached the Senate intelligence committee this summer with a request: They wanted the transcript of an interview Senate staff had conducted with former Trump campaign chairman Paul Manafort.

But they were blocked. Manafort's lawyers said they had not permitted Mueller's legal team to access the interview transcript under the agreement with the committee on conferences, even though Mueller's attorneys said they had been given permission. The matter is still under discussion, sources say.

The previously undisclosed oppose, described to CNN by multiple sources, underscores the new challenges as congressional committees and Mueller's operation head into a more intense stage of their parallel — and sometimes, conflicting — investigations into Russian election meddling and any collusion with Trump associates.