(Kevin Lamarque/Reuters)

On Sunday morning, President Trump expressed frustration that his campaign is under investigation over possible ties to Russia's plot to influence the 2016 election but that his former opponent Hillary Clinton is not facing the same level of scrutiny.

In four tweets sent over 24 minutes, Trump wrote: “Never seen such Republican ANGER & UNITY as I have concerning the lack of investigation on Clinton made Fake Dossier (now $12,000,000?), the Uranium to Russia deal, the 33,000 plus deleted Emails, the Comey fix and so much more. Instead they look at phony Trump/Russia, 'collusion,' which doesn't exist. The Dems are using this terrible (and bad for our country) Witch Hunt for evil politics, but the R's are now fighting back like never before. There is so much GUILT by Democrats/Clinton, and now the facts are pouring out. DO SOMETHING!”

The tweets came as CNN has continued to report that on Friday a federal grand jury in Washington approved the first charges in the investigation led by special counsel Robert S. Mueller III, citing “sources briefed on the matter.” The charges are sealed, and it's unclear who could be charged and for what.

The history of the dossier Trump referenced is a complicated one. During the 2016 Republican primaries, the conservative Washington Free Beacon paid the Washington firm Fusion GPS to investigate Trump's background and, eventually, his business ties to Russia, The Washington Post reported Friday. The publication receives financial support from billionaire GOP donor Paul Singer, according to two people familiar with Singer, whose firm did not respond to requests seeking comment.

Fusion GPS is an opposition research firm run by ex-journalists, but how is it connected to the Trump dossier, Donald Trump Jr.'s Trump Tower meeting and the 2016 election? The Fact Checker's Glenn Kessler explains. (Meg Kelly/The Washington Post)

After the Free Beacon stopped paying Fusion GPS, the research firm offered in April 2016 to continue researching Trump for the Clinton campaign and the Democratic National Committee, The Post reported last week. According to that reporting, Fusion GPS then hired Christopher Steele, a former British intelligence officer with ties to the FBI and the U.S. intelligence community, who compiled a 35-page dossier on Trump. Brian Fallon, a former spokesman for the Clinton campaign, said he learned about Steele and the dossier after the election. People familiar with the matter said the Clinton campaign and the DNC did not direct Steele’s activities.

It's unclear how much the Clinton campaign and DNC paid for the information. The Clinton campaign paid Perkins Coie $5.6 million in legal fees from June 2015 to December 2016, according to campaign finance records cited by The Post, and the DNC paid the firm $3.6 million in “legal and compliance consulting’’ since November 2015 — though it’s impossible to tell from the filings how much of that work was for other legal matters and how much of it related to Fusion GPS. It's unclear how Trump arrived at the $12 million figure used in his tweets.

When it comes to the “Uranium to Russia deal” that Trump referenced in his tweet, House and Senate Republican leaders have already announced two investigations into decisions made during the Obama administration, when Clinton was secretary of state, involving a uranium deal that increased Russia’s share of the U.S. nuclear market. At the same time, the House Oversight Committee has launched a probe into the Clinton email investigation that will examine, among other things, why then-FBI Director James B. Comey publicly announced that the bureau was investigating Clinton but then waited months before making a similar announcement about its inquiries into the Trump campaign.

Democrats have argued that these investigations are intended to distract from the probe into Russian interference in the 2016 election.