In this explainer, Cindy L. Otis — who served as an intelligence analyst at the Central Intelligence Agency from 2007 to 2017 — describes how intelligence analysts make assessments, like that which found Russia had interfered in the U.S. presidential election last year.

During his visit to Europe for the G-20 meeting earlier this month, President Donald Trump again rejected the United States Intelligence Community’s (IC) assessment from January that found Russia interfered in the 2016 presidential election.

On the margins of G-20, he met with Russian president Vladimir Putin and afterward tweeted that the U.S. needed to move on constructively with Russia, because Putin had reportedly denied any Russian involvement in the U.S. election in the meeting. He confirmed reports that he and Putin also discussed establishing a joint “impenetrable cyber unit” to prevent future election hacking — an outrageous proposal, considering that doing so would allow the same country that influenced the U.S. election access to our cyber capabilities and expose us to additional election hacking. Trump later withdrew the proposal after it sparked criticism from both Democrats and Republicans.

So what did the IC actually determine in its assessment about Russia’s meddling in the 2016 election? Here’s everything you need to know about the intelligence professionals tasked to analyze Russia’s interference, how these reports are made, and why they matter.

What is the role of intelligence analysts?

Intelligence analysis is like putting together a puzzle without all of the pieces, and then trying to describe for policymakers the picture. Analysts are federal employees responsible for anticipating and assessing rapidly evolving issues across the globe that may have both positive and negative consequences for U.S. national security and foreign policy interests. The actual number of employees who work as analysts in the IC is classified. Analysts look specifically at political, security, economic, leadership, and terrorism issues worldwide, including domestic threats posed by foreign persons. The FBI is the only member of the IC with law enforcement responsibilities and the ability to investigate U.S. citizens.

Intelligence analysts provide non-partisan, strategic assessments to government policymakers in written products, most notably, the President’s Daily Brief (PDB). CIA director Mike Pompeo said in January that Trump is a “serious consumer” of intelligence, but his PDB is much shorter than those for the past two presidents.

Not all of the government’s intelligence agencies have an intelligence analysis arm, as most focus on intelligence collection against threats overseas instead. For agencies like the CIA, their primary customer is the president, but they also support the military, diplomats, Congress, and other officials at the executive branch level.

Which government agencies are part of the Intelligence Community?

There are 17 agencies in the Intelligence Community. They include: Air Force Intelligence, Army Intelligence, Central Intelligence Agency, Coast Guard Intelligence, Defense Intelligence Agency, Energy Department, Homeland Security Department, State Department, Treasury Department, Drug Enforcement Administration, Federal Bureau of Investigation, Marine Corps Intelligence, National Geospatial Intelligence Agency, National Reconnaissance Office, National Security Agency, Navy Intelligence, and the Office of the Director of National Intelligence.

Which intelligence agencies looked into Russian interference in the election?

Normally, the ODNI chooses one to several IC agencies to draft an Intelligence Assessment and then coordinates that assessment with the rest of the IC. However, due to the highly classified nature of Russian involvement in the election, the assessment was written by “two-dozen” officers from the Central Intelligence Agency (CIA), Federal Bureau of Investigation (FBI), and the National Security Agency (NSA) that were “hand-picked, seasoned” analysts, according to testimony given by former director of national intelligence James Clapper to the Senate Judiciary committee in May.