WASHINGTON (AP) — The House Intelligence Committee chairman is threatening to subpoena FBI Director Christopher Wray for information related to the bureau's counterintelligence investigation into Russian interference in the 2016 U.S. presidential election.

Rep. Adam Schiff, D-Calif., said Wednesday that he has unsuccessfully sought more information about that investigation and any links to Donald Trump's winning campaign, including whether that probe is still active. The investigation was first disclosed by then-FBI Director James Comey at a committee hearing in March 2017, and Schiff said he has received few answers about it since Comey was fired by Trump two months later.

While special counsel Robert Mueller did examine Russian interference and possible ties to the Trump campaign, Schiff wants to know whether the FBI is still conducting any related counterintelligence investigations. Such inquiries can take years and extend far beyond a criminal investigation.

"We are determined to get answers, and we are running out of patience," Schiff said after a hearing on the counterintelligence implications of Mueller's report. "If necessary, we'll subpoena the director and require him to come in and provide those answers under oath."

At the hearing, one of several that House Democrats are holding to shed more light on the Mueller report, former FBI officials told lawmakers that Russian meddling bore some of the textbook tricks of the trade of Kremlin spycraft, including the volume and breadth of contacts with Trump associates.

The two witnesses, Robert Anderson and Stephanie Douglas, highlighted aspects of the Mueller report they said showed Russian efforts to screen and test Trump campaign associates, to establish back channels of communications and to spread their contacts around in hopes of maximizing their chances of getting what they wanted.

"It is an absolute classic tradecraft of Russia and Russian intelligence services. They'll never have one point of failure," said Anderson, a former FBI executive assistant director who used to supervise counterintelligence investigations. "If they're looking to try to obtain or pass information or potentially even influence information, they'll make sure that they have numerous aspects or points to where they can try to get that done."

Mueller did not find a criminal conspiracy between the campaign and Russia, but he did detail a series of interactions and outreach that have alarmed Democrats and accelerated calls from some in the party for impeachment proceedings and renewed investigations.

Schiff noted that Mueller detailed more than 100 contacts between Russia and associates of the president.

Among the interactions was a June 2016 meeting at Trump Tower during which the president's oldest son, Donald Trump Jr., expected to receive dirt on his father's Democratic opponent, Hillary Clinton. Trump's first national security adviser, Michael Flynn, discussed Obama administration sanctions against Russia with the then-Russian ambassador in the weeks leading up to the president's inauguration in January 2017.

"It immediately put the existing administration in a horribly conflicting position, and they didn't know about the backchannel in advance of the inauguration," said Douglas, a former FBI executive assistant director. "It also probably assured the Russians that they were going to get a more favorable treatment" by the incoming Trump administration.

She highlighted how Trump's campaign chairman, Paul Manafort, shared polling data during the campaign with Konstantin Kilimnik, a business associate accused of having ties to Russian intelligence. Manafort also told Kilimnik he was willing to provide "private briefings" about the Trump campaign to Oleg Deripaska, a Russian billionaire with ties to Russian President Vladimir Putin.

"I thought that that was very interesting, that they are tasking him and building upon that," Douglas said. "And if he would have stayed with the campaign, I am sure they would have continued to task him."

Also Wednesday, Trump Jr. spoke with the Senate Intelligence Committee for about three hours to clarify an interview with the committee's staff in 2017. Senators wanted to talk to him again about the Trump Tower meeting and a Trump real estate project in Moscow.

The president's former lawyer, Michael Cohen, told a House committee in February that he had briefed Trump Jr. approximately 10 times about a plan to build a Trump Tower in Moscow before the 2016 election. But Trump Jr. had told Congress he was only "peripherally aware" of the real estate proposal.

As he left the interview, Trump Jr. said he was happy to clarify his answers, but "I don't think I changed any of what I said because there was nothing to change."

The two ex-FBI officials who testified Wednesday retired from the bureau before it opened its investigation into the Trump campaign in summer 2016. By inviting them instead of agents involved in the investigation, Democrats are giving center stage to longtime career officials devoid of the political baggage that accompanies some of the Republican president's more outspoken critics.

Even so, the partisan divisions surfaced again. Schiff said most Americans consider the Trump campaign's embrace of Russian aid "to constitute plain evidence of collusion." The committee's top Republican, Rep. Devin Nunes of California, described allegations of collusion to be a "hoax" and suggested Democrats should apologize.

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Associated Press writer Laurie Kellman in Washington contributed to this report.