WASHINGTON — President Trump has given Attorney General William P. Barr extraordinary powers to declassify intelligence secrets as part of his review into how the 2016 Trump campaign’s ties to Russia were investigated. That means Mr. Barr could make documents or information from the C.I.A. or the F.B.I. public over their intense objections, setting up a possible confrontation between the law enforcement and intelligence communities.

As the president tries to find evidence that he was the target of a political witch hunt, former and current intelligence officials are worried about the exposure of secret sources and sensitive methods. “This was an attempted takedown of the president of the United States,” Mr. Trump said on Friday.

Here is what we know about the origins of the investigation.

Why did the F.B.I. investigate?

In July 2016, WikiLeaks released Democratic emails stolen by Russian military intelligence officers and posted thousands of internal Democratic National Committee documents revealing information about the Clinton campaign. That same month, the F.B.I. learned that a Trump campaign adviser, George Papadopoulos, had told an Australian diplomat that he was told that the Russians had stolen Democratic emails before they were made public. F.B.I. agents traveled to London to interview the diplomat and his assistant.

Those interviews, along with information about Russian hacking, were used to open the F.B.I.’s investigation into whether any Trump associate had conspired with the Russian government. On Friday, Mr. Trump said he hoped that Mr. Barr would scrutinize the roles of the Australian and British governments in the opening of the F.B.I.’s Russia investigation. Both countries work closely with the F.B.I. and the C.I.A.