The White House has made significant progress turning over documents to Bob Mueller’s special counsel probe, according to a lawyer familiar with the investigation. The lawyer also said that several current and former White House staffers either have been interviewed by Mueller’s investigators or are scheduled to be interviewed by them in the next few weeks.

Mueller’s powerhouse team of investigators is running, among other things, a counterintelligence investigation tasked with looking at any coordination or links between Trump campaign associates and the Russian government. They are also tasked with looking at Russia’s broader efforts to meddle in the 2016 campaign, according to the order Deputy Attorney General Rod Rosenstein signed to authorize their probe.

As part of the investigation, Mueller’s team requested a host of documents from the White House, as has been reported.

The attorney familiar with the probe told The Daily Beast that the White House has made substantial progress responding to Mueller’s requests.

“The bulk of the heavy lifting is largely behind,” the lawyer said.

But it isn’t finished. Mueller and his investigators have been making follow-up requests of the White House legal team as they follow up on their original queries and refine them, per our source, and the legal team has been moving to answer those requests as they come.

A spokesman for Mueller declined to comment for this story.

So far, media reports have only identified one White House staffer whom Mueller’s team has interviewed: Fox News reported that Mueller’s team interviewed retired Lt. Gen. Keith Kellogg on Sept. 28. Kellogg is the chief of staff for the National Security Counsel.

The Washington Post reported last month that a number of current and former White House officials are expected to face interviews from Mueller’s team, including Justice Department attorney James Burnham, former press secretary Sean Spicer, former chief of staff Reince Priebus, current communications director Hope Hicks, White House counsel Don McGahn, and White House spokesman Josh Raffel.

Meanwhile, congressional investigations into potential collusion between Team Trump and Russian government actors are also continuing apace. The Daily Beast reported this week that Cambridge Analytica, a data firm backed by the pro-Trump billionaire megadonors Robert and Rebekah Mercer, is turning over documents to the House Permanent Select Committee on Intelligence.

As those investigations hum along, a clearer picture of Russia’s efforts to influence the 2016 election is emerging. The Daily Beast has reported extensively on how exactly Russian actors used Facebook groups and YouTube videos to try to persuade voters to back Trump—or, in some cases, just to stay home.