With four main investigations into Russian election interference and possible Trump campaign collusion happening simultaneously, it is difficult to keep track of who is testifying and who is scheduled to meet with investigators in the coming weeks.

Complicating matters is the fact that neither Congress nor the Special Counsel regularly reveals interview plans.

This past week, Donald Trump Jr. met with the Senate Intelligence Committee behind closed doors for nine hours Wednesday. Afterwards, Vice Chairman Mark Warner (D-VA) said he would like to see the president's son return to answer questions from the full committee.

On Thursday, House Intelligence Committee members interviewed Cambridge Analytica's Alexander Nix via videoconference.

NYT:

Trump's campaign hired Alexander Nix and the company, Cambridge Analytica, in June 2016 and paid it more than $6.2 million through last December, according to Federal Election Commission records. The month after his firm was hired, Nix emailed WikiLeaks founder Julian Assange for help tracking down some 33,000 emails that Clinton supposedly had deleted from her private server... Nix wanted to convert the missing and potentially damaging emails into a searchable database for use by the Trump campaign or a pro-Trump political action committee...

Also Thursday, Congressional investigators reportedly spoke with British publicist Rob Goldstone, but The Washington Post does not specify which committee conducted the interview.

Special Counsel Robert Mueller's investigators met with White House Counsel Don McGahn earlier this week which Trump attorney Ty Cobb claims is the last of the White House interviews.

CNN:

He was among the last of the White House employees to be interviewed, marking what Trump's legal team views as an important milestone in the ongoing Russian meddling investigation. "The White House interviews are completed," Cobb told CNN Tuesday. "We remain hopeful for an appropriate and prompt conclusion."

This coming week, Trump's personal lawyers will meet with Mueller and his team, which, also according to CNN, will be "an opportunity to gain a clearer understanding of the next steps in Mueller's probe."

And House Intelligence Committee staffers are traveling to New York to interview Rhona Graff, Donald Trump's longtime personal assistant, and Felix Sater, a former Trump business associate and convicted criminal. It's an unusual move that committee Democrats suspect is designed to shut them out and help Republicans close out the investigation as quickly as possible.

CNN:

In addition to Sater and Graff, the panel plans to interview two of its colleagues next week -- GOP Rep. Dana Rohrabacher of California, who has friendly ties with Russia, and Democratic Rep. Debbie Wasserman Schultz of Florida, the former head of the Democratic National Committee, which last year paid a law firm that retained Fusion GPS, the firm behind the Trump-Russia dossier.

Trump lawyers set for key meeting with special counsel next week (CNN)

House Committee Questions Head of Trump Campaign Data Firm: Sources (NYT)

DONALD TRUMP JR. IN MARATHON NINE-HOUR INTERVIEW OVER RUSSIA ELECTION INTERFERENCE (Newsweek)

Music promoter dangled possible Putin meeting for Trump during campaign (WaPo)

House intel heading to NY for two key interviews, prompting grumbling from Democrats (CNN)