In May 2016, a Russian military intelligence officer talked too much. Boasting to a colleague, he said that his organization, known as the GRU, was getting ready to cause chaos in the upcoming U.S. presidential election. The officer was “bragging about the systematic attempt... to cause chaos into our electoral cycle,” a senior U.S. intelligence official told TIME for the magazine's current cover story on the Russian operation.

What the officer didn’t know was that U.S. spies were listening. Looking back as part of their effort to uncover the details of the 2016 Russia operation, U.S. investigators now realize the GRU officer’s boast was the first indication they had from their sources that Russia wasn’t just hacking U.S. email accounts to collect intelligence, but was actually planning to interfere in the vote, several senior intelligence officials told TIME.

A year later, that single scrap of intelligence has grown into a multi-pronged investigation to uncover the full extent of Russia’s election-meddling, an attack on the core exercise of democracy in America. The probes have uncovered the possibility that members of Donald Trump's presidential campaign cooperated with Moscow’s agents, according to the ousted head of the FBI, James Comey, and other senior Justice Department and intelligence officials. Intelligence collected last year, former CIA Chief John Brennan testified on May 23, "raised questions in my mind, again, whether or not the Russians were able to gain the cooperation of those individuals." The acting FBI director, Andrew G. McCabe, confirmed in testimony earlier this month that the bureau was pursuing a “highly significant” investigation into possible collusion.

The FBI investigators don't know—or haven't said—whether the Trump campaign helped the Russians. Determining whether the President's campaign colluded is up to independent, professional investigators, overseen by prosecutors and judges, who are following the facts to uncover the truth.

Which is why Trump’s multiple attempts to blunt the work of the investigators is so remarkable.

At a White House meeting on Feb. 14, according to the New York Times , Trump asked Comey to ease off part of the Russia probe. Comey refused. When Comey testified March 20 that the FBI was looking into potential cooperation between the Trump campaign and Russia, the President and White House aides asked the heads of the NSA and DNI to pressure Comey to back off, according to the Washington Post . When none of that worked, Trump fired Comey on May 9.

Trump said May 18 that the "entire" FBI investigation is a “witch hunt.” He reportedly told the Russian foreign minister and others in a White House meeting May 10 that Comey, who has held multiple senior jobs in the Justice Department over more than 30 years, was “a nut job.” But Trump is increasingly alone in his assessment that the investigation is in any way improper. “These investigations are in place to get us to the right conclusion,” Trump's Director of National Intelligence, Dan Coats testified on May 23.

And they are making progress. Initially, in May 2016, the report on the GRU officer didn’t jump out. “We could not put a value on it,” says the senior U.S. intelligence official, “because we didn’t really understand the context until much later.” But over the following two months, new reports came in from other channels showing it wasn’t just the GRU; other elements of the Russian government were intending to interfere in the election, too. By the end of July, there was sufficient evidence to indicate a serious Russian operation was underway.

It was at that point that the FBI, which investigates whether or not crimes or foreign espionage have taken place in the United States, opened its probe of the Russian operation. The bureau brings broad powers to such investigations. They can subpoena documents, require testimony by suspects or witnesses under oath, and eavesdrop on suspects to collect evidence. All of that work must comply with the stringent rules of the Domestic Investigations and Operations Guide .

The FBI is reportedly looking at the activities of several individuals associated with the Trump campaign, including former campaign chair Paul Manafort and two advisers, Roger Stone and Carter Page. Each has denied doing anything wrong. Investigators are also looking into contacts between the Russians and Trump's ousted National Security Advisor Michael Flynn. The FBI now views a current senior White House official as a significant person of interest in the case, according to the Washington Post .

The DIOG imposes strict oversight constraints on the bureau's agents, especially when investigating politically sensitive matters. To issue subpoenas they need the permission of a prosecutor or grand jury. To pursue particularly sensitive targets, like political figures, they must get approval from top Justice Department officials. If they want to eavesdrop on suspects, they generally must convince a judge they have probable cause to believe a crime was committed.

Public testimony since the election indicates that FBI investigators, following the evidence and working under the supervision and scrutiny of U.S. prosecutors and independent judges, crossed that threshold months ago. Regardless of who is in charge at the top of the FBI or the Justice Department, the probe is going to continue until investigators determine the full extent of the Russian operation, and whether any Americans helped them.