WASHINGTON – The day has finally come.

Special counsel Robert Mueller has completed his investigation into Russian interference into the 2016 election and possible collusion with President Donald Trump's campaign. In announcing the end of the investigation, Attorney General William Barr sent a letter to members of Congress informing them that the probe, which roots go back to summer 2016, had been completed.

"I write to notify you ... that special counsel Robert S. Mueller III has concluded his investigation of Russian interference in the 2016 election and related matters," Barr wrote in the letter, which was sent to the top Democrats and Republicans on the Senate and House Judiciary Committees.

Barr said in the letter that Mueller had submitted his final, confidential report, and that he could alert lawmakers in the coming days to its findings. Those findings will likely be made public at the same time, according to Justice Department spokeswoman Kerri Kupec.

More:Special counsel Robert Mueller delivers report marking end of investigation into Trump's campaign, Russia

Barr told lawmakers he is "reviewing the report, and anticipate that I may be in a position to advise you of the special counsel's principal conclusions as soon as this weekend."

But for now, Mueller's findings remain a mystery.

In the nearly two years since Mueller was installed to oversee that work, the investigation has resulted in charges against more than two dozen Russian operatives, and a succession of aides and advisers to Trump's campaign, including its former chairman. In doing so, prosecutors mapped the details of a Russian operation to sway the election, and a campaign eager to reap the benefit of those efforts.

Read the full letter here:

Dear Chairman Graham, Chairman Nadler, Ranking Member Feinstein, and Ranking Member Collins: I write to notify you pursuant to 28 C.F.R. 8 600.9(a)(3) that Special Counsel Robert S. Mueller III has concluded his investigation of Russian interference in the 2016 election and related matters. In addition to this notification, the Special Counsel regulations require that I provide you with "a description and explanation of instances (if any) in which the Attorney General" or acting Attorney General "concluded that a proposed action by a Special Counsel was so inappropriate or unwarranted under established Departmental practices that it should not be pursued." 28 C.F.R. 8 600.9(a)(3) 9. There were no such instances during the Special Counsel's investigation The Special Counsel has submitted to me today a "confidential report explaining the prosecution or declination decisions" he has reached, as required by 28 C.F.R. 8 600.8(c). I am reviewing the report and anticipate that I may be in a position to advise you of the Special Counsel's principal conclusions as soon as this weekend 99 prosecution or declination decisions Separately, I intend to consult with Deputy Attorney General Rosenstein and Special Counsel Mueller to determine what other information from the report can be released to Congress and the public consistent with the law, including the Special Counsel regulations, and the Department's long-standing practices and policies. I remain committed to as much transparency as possible, and I will keep you informed as to the status of my review. Finally, the Special Counsel regulations provide that "the Attorney General may determine that public release of" this notification "would be in the public interest." 28 C.F.R. { 600.9(c). I have so determined, and I will disclose this letter to the public after delivering it to you. Sincerely, William P. Barr Attorney General

Contributing: Kevin Johnson