WASHINGTON (AP) — In a London bar three years ago, a young foreign policy adviser for Donald Trump's campaign told an Australian diplomat something astonishing: Russia had dirt on Hillary Clinton.

Within months, the adviser, George Papadopoulos, was proven right as stolen emails damaging to Clinton surfaced online. Alarmed Australian officials tipped off their American counterparts.

For the FBI, the information was worrisome. How did Papadopoulos get advance word of the emails? Was he or anyone else in the Trump campaign working with the Russians?

Those questions led to the Russia investigation that would shadow Trump's presidency for nearly two years. Trump and his supporters have seized on the FBI's use of informants in Papadopoulos' case and surveillance of another campaign aide to accuse the Justice Department and the FBI of unlawfully spying on his campaign.

But both are common investigative techniques for agents trying to determine whether a foreign adversary is trying to compromise American national security. The use of informants and surveillance is not illegal, if proper procedures were followed, a point Attorney General William Barr made this week in congressional testimony.

A look at what we know about the surveillance of Trump's campaign.

PAPADOPOULOS

A few months after Papadopoulos' discussion with the Australian diplomat, a professor named Stefan Halper emailed him offering $3,000 to come to London. Halper said it would be to discuss a natural gas project in the Mediterranean, calling Papadopoulos a "recognized expert in this field."

Unbeknownst to Papadopoulos, Halper was actually a longtime FBI informant, several news outlets, including The Washington Post, The Wall Street Journal and NBC News, have reported.

On Thursday, The New York Times reported that a woman who identified herself as Halper's assistant was a government investigator working for the FBI.

In his book about his entanglement in the Russia probe, "Deep State Target," Papadopoulos wrote that the woman, who identified herself as Azra Turk, asked him about his work with the Trump campaign.

"She wants to know: are we working with Russia?" he wrote.

He described her question as "creepy" and said he told her he had "nothing to do with Russia."

Papadopoulos was later questioned by the FBI and ultimately pleaded guilty to lying to federal agents about his conversations with another professor, who in March 2016 had told him about the Clinton "dirt."

Papadopoulos was sentenced to 14 days behind bars and was the first Trump aide to plead guilty and agree to cooperate in special counsel Robert Mueller's Russia investigation.

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Former Trump campaign foreign policy adviser Carter Page had been on the FBI's radar as early as 2013, when the bureau suspected Russian intelligence agents were trying to recruit Americans.

And when he surfaced during the 2016 campaign as an adviser to Trump — and traveled to Russia — he once again attracted the attention of the FBI as it investigated Russian election interference.

In October 2016, after Page had left the Trump campaign, the FBI obtained a warrant to monitor him under the Foreign Intelligence Surveillance Act, which allows the government to conduct surveillance in some of the most sensitive FBI investigations.

In the FISA application, the government said Page "has been the subject of targeted recruitment by the Russian Government" and said the FBI believed Russia's efforts to influence the 2016 election "are being coordinated with Page and perhaps other individuals associated with" Trump's campaign."

Some of the since-declassified FISA application relied on a collection of memos authored by former British spy Christopher Steele that contained uncorroborated allegations of ties between Trump and his associates and Russia.

Those documents, known colloquially as the Steele Dossier, contend the Russian government amassed compromising information about Trump and had been engaged in a years-long effort to support and assist him.

Steele's research was funded by Clinton's campaign through a Washington law firm, a fact congressional Republicans have seized on as they criticized the Russia investigation. They argued the dossier was inappropriately used by the Justice Department before the election to get the FISA warrant on Page and claimed the FBI and the Justice Department didn't tell the FISA court enough about the political money behind Steele's efforts.

Trump has repeatedly criticized the use of the dossier, claiming, incorrectly, that it started the Russia investigation.

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WAS ANY OF THIS ILLEGAL?

Based on available public information, there's no evidence of illegal activity by the FBI or the Justice Department.

The FBI routinely uses informants, also known as confidential sources, in a variety of investigations, from violent crimes to white-collar and counterintelligence investigations.

The FISA warrant the FBI obtained to monitor Page was lawfully obtained from a court in October 2016. A memo released by House Republicans in 2018 showed that a judge agreed four times that there was probable cause to believe Page was acting as an agent of a foreign power. Top FBI and Justice Department officials, including Deputy Attorney General Rod Rosenstein, signed off on the FISA application.

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WHAT HAPPENS NOW?

Barr has said he's compiled a team to look into the origins of the Russia investigation, a matter that is already being investigated by the department's inspector general.

In his testimony during a congressional budget hearing last month, Barr said he believes "spying did occur" on the campaign and suggested the origins of the probe may have been mishandled.

Barr provided no details about what "spying" may have taken place but appeared to be referring to Page. Barr has said he doesn't see the word as pejorative and stopped short of saying anything unlawful or improper took place.

For his part, Trump has repeatedly accused the government of committing an illegal, unprecedented act. Senate Republicans have said they will also investigate the origins of the Russia investigation.