The Department of Justice’s investigation into the origins of the 2016 Russia probe is “broad in scope and multifaceted” and will examine potential involvement of “foreign intelligence services,” a top agency official said on Monday.

In a letter to Congress, Assistant Attorney General Stephen Boyd provided an outline of the investigation to House Judiciary Committee Chairman Jerry Nadler (D-NY) after the congressional leader’s inquiry into the review.

Attorney General William Barr said last month that he had directed John Durham, the United States attorney in Connecticut and a veteran prosecutor, to determine if law enforcement and intelligence authorities engaged in improper surveillance as they investigated potential coordination between Russia and the Trump campaign to sway the 2016 presidential election.

“It is now well-established that, in 2016, the U.S. government and others undertook certain intelligence-gathering and investigative steps directed at persons associated with the Trump campaign,” said the letter from Boyd, the department’s top liaison to Congress.

“As the Attorney General has stated publicly at congressional hearings and elsewhere, there remain open questions relating to the origins of this counter-intelligence investigation and the U.S. and foreign intelligence activities that took place prior to and during that investigation,” the letter said.

The point of the review, Boyd added, “is to more fully understand the efficacy and propriety of those steps” and to answer open questions for the attorney general.

Further, Boyd revealed Durham was reviewing potential involvement of foreign intelligence services. Former Trump aide George Papadopoulos told Fox News last month that an informant who was likely “CIA and affiliated with Turkish intel” had posed as a Cambridge University research assistant in September 2016 and tried to “seduce him” to obtain information linking the Trump team to Russia,” Fox News notes.

Durham is “very dialed in” to the sweeping review, Fox News has also reported, and has met with Barr “on multiple occasions in recent weeks” in Washington, D.C. He is asking “all the right questions,” Justice Department sources told the news outlet.

Barr has repeatedly said he believes there was “spying” on the Trump campaign. Testifying before Congress, Barr said he believed “spying did occur” on the Trump’s campaign. “I think spying on a political campaign is a big deal,” the nation’s chief legal officer testified before the Senate Appropriations Committee, before adding, “I am not suggesting that those rules were violated, but I think it’s important to look at them.”

Last month, President Trump declared his campaign was illegally surveilled, accusing officials responsible of treason. “My Campaign for President was conclusively spied on,” the president tweeted. “Nothing like this has ever happened in American Politics. A really bad situation. TREASON means long jail sentences, and this was TREASON!”

The Associated Press contributed to this report.