In a secure meeting room under the Capitol last week, legislators held in their hands a classified letter written by colleagues in the Senate summing up a secret, new CIA assessment of Russia's role in the 2016 presidential election.

Sitting before the House Intelligence Committee was a senior FBI counterintelligence official. The question the Republicans and Democrats in attendance wanted answered was whether the bureau concurred with the conclusions the CIA had just shared with senators that Russia "quite clearly" intended to help Republican Donald Trump defeat Democrat Hillary Clinton and clinch the White House.

For the Democrats in the room, the FBI's response was frustrating. During a similar Senate Intelligence Committee briefing held the previous week, the CIA's statements, as reflected in the letter, were "direct and bald and unqualified" about Russia's intentions to help Trump, according to one of the officials who attended the House briefing.

The FBI official's remarks to the legislators were, in comparison, "fuzzy" and "ambiguous," suggesting to those in the room that the bureau and the agency weren't on the same page, the official said.

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The divergent messages from the CIA and the FBI put a spotlight on the difficulty faced by intelligence and law enforcement officials as they try to draw conclusions about the Kremlin's motives for hacking Democratic Party emails during the 2016 race. Officials are frequently looking at information that is fragmentary. They also face issues assessing the intentions of a country expert at conducting sophisticated "influence" operations that made it hard - if not impossible - to conclusively detect the Kremlin's elusive fingerprints.

The competing messages, according to officials in attendance, also reflect cultural differences between the FBI and the CIA. The bureau, true to its law enforcement roots, wants facts and tangible evidence to prove something beyond all reasonable doubt. The CIA is more comfortable drawing inferences from behaviour.

"The FBI briefers think in terms of criminal standards - can we prove this in court," one official said. "The CIA briefers weigh the preponderance of intelligence and then make judgment calls to help policymakers make informed decisions. High confidence for them means 'we're pretty damn sure.' It doesn't mean they can prove it in court." The FBI is not sold on the idea that Russia had a particular aim in its meddling. "There's no question that [the Russians'] efforts went one way, but it's not clear that they have a specific goal or mix of related goals," said one official.