How did American intelligence officials come to brief President Obama, President-elect Donald J. Trump and lawmakers about supposed Russian plans to try to blackmail Mr. Trump? There are far more questions than answers. But here is a look at the story so far.

WHAT WE KNOW

Last year, a D.C. political research firm, paid by Trump’s Republican rivals, hired a retired British intelligence officer to investigate the candidate’s ties to Russia.

After it became clear that Mr. Trump would be the Republican nominee, Democratic clients began to pay the firm for this same “opposition research,” standard practice in politics.

The former British spy, who had long experience in Russia and a network of connections there, compiled dozens of reports detailing what he heard from his contacts. The memos he wrote, mostly 1-3 pages long, are dated from June to December.

The memos contain unsubstantiated claims that Russian officials tried to obtain influence over Mr. Trump by preparing to blackmail him with sex tapes and bribe him with business deals. They also claim that the Trump campaign met with Russian operatives to discuss the Russians’ hacking and their leaking of emails and documents from the Democratic National Committee and from Hillary Clinton’s campaign chairman, John Podesta.

The Washington firm and the former British spy, not identified here because of a confidential source agreement with The NYTimes, gave the memos first to their clients but later to the FBI and multiple journalists at The Times and elsewhere. The memos, totaling about 35 pages, also reached several members of Congress.

Last week, when the FBI, CIA and National Security Agency gave a classified report on the Russian hacking and leaking and efforts to influence the presidential election to Mr. Obama, Mr. Trump and congressional leaders, they attached a 2-page summary of the unverified allegations in the memos.

WHAT WE DON’T KNOW

Whether any of the claims in the memos are true. American intelligence agencies have not confirmed them, and Mr. Trump has said they are a complete fabrication. In addition, one specific allegation - that Mr. Trump’s lawyer, Michael Cohen, met with a Russian official in Prague in August or September - has been denied by both Mr. Cohen, who says he has never been to Prague, and the Russian, Oleg Solodukhin.

Who concocted the information in the memos, if it is entirely false or partially so, and with what purpose. Did the British intelligence officer accurately report what he heard? Who gave him the information that, if false, amounts to a very sophisticated fabrication?

What exactly prompted American intelligence officials to pass on a summary of the unvetted claims to Mr. Obama, Mr. Trump and Congress? Officials have said they felt the president-elect should be aware of the memos, which had circulated widely in Washington. But why put the summary in a report going to multiple people in Congress and the executive branch, virtually assuring it would be leaked?

What will happen now. The FBI has been investigating the claims in the memos, and Democrats are demanding a thorough inquiry into the reports that Trump representatives met with Russian officials during the campaign. But as of Jan. 20, Mr. Trump will be in charge of the bureau and the other intelligence agencies, and he may not approve such an investigation.