WASHINGTON — For more than a year, congressional Democrats have pushed to unlock a mystery they said could reveal whether Donald Trump had advance knowledge of a meeting his eldest son had in the summer of 2016 with a Russian lawyer promising damaging material about Democratic candidate Hillary Clinton.

The key, they said, was to determine who was using blocked telephone numbers to speak to Donald Trump Jr. before and after the meeting in Trump Tower, suggesting he may have communicated with his father on a private line.

But Senate investigators have concluded the blocked calls were conversations between Trump Jr. and two longtime business associates: Howard Lorber, a real estate investor who has done business in Russia, and Brian France, the chief executive of NASCAR, according to two people familiar with the determination.

The findings were first reported by CNN and ABC News.

France did not immediately respond to a request for comment.

A spokesman for Lorber said Friday that the real estate magnate “does not recall conversations with Donald Trump Jr. in the summer of 2016,” adding that Lorber never discussed “any Russian matters” with the president’s son.

Lorber is a longtime friend of the elder Trump, and the two went to Moscow in 1996 to scout for a real estate deal that never materialized, according to people familiar with the visit. The trip helped spur Trump’s interest in seeking other ways over the years to build a tower in Moscow.

President Trump has long denied he knew about the Trump Tower gathering in 2016, which was attended by Trump Jr., his brother-in-law Jared Kushner and Trump campaign chairman Paul Manafort. At the meeting, Trump Jr. was expecting a Russian lawyer linked to the Kremlin to share dirt on Clinton, he later told Congress, but she instead pressed him on a U.S. law levying sanctions on Russia for human rights abuses.

The president and his allies seized on conclusions by congressional investigators, saying the findings undercut Democratic efforts to raise questions about links between Trump and Russia’s interference in the 2016 campaign.

“The big deal, very mysterious Don jr telephone calls, after the innocent Trump Tower meeting, that the media & Dems said were made to his father (me), were just conclusively found NOT to be made to me. ... Really sad,” the president tweeted late Thursday.

“This Witch Hunt must end!” he added.

In his own tweet, Trump Jr. wrote: “More of the Democrat’s #fakenews narrative disappearing before their eyes. I wonder how many more false leaks will pop now to keep their dreams alive???”

Trump Jr. told the Senate Judiciary Committee in 2017 that his father had no knowledge of the June 2016 meeting with the Russians.

Democrats, led by Rep. Adam Schiff, D-Calif., who now chairs the House intelligence committee, were skeptical of Trump Jr.’s account, noting that his first explanations of the meetings were misleading.

In a July 8, 2017, statement to The New York Times, Trump Jr., said meeting participants “primarily discussed a program about the adoption of Russian children.” The statement did not mention that he was promised damaging information about Clinton — or that Trump Jr. had responded enthusiastically to the offer.

“If it’s what you say I love it,” he wrote in an email to an associate who offered to set up the meeting.

Democrats raised questions about phone calls made by Trump Jr. on June 6, 2016, that they thought might show that Trump’s son informed his father about the meeting in advance.

At 4:04 p.m. that day, phone records show, Emin Agalarov, a wealthy Russian who helped arrange the meeting, called Trump Jr.

Phone records show Trump Jr. called Agalarov at 4:31 p.m. Between those two calls, phone records show that Trump Jr. was connected with a blocked phone number.

Trump Jr. told the Senate Judiciary Committee that he did not remember whom he spoke with at that time, but others testified at the time that his father uses a blocked line.

Democratic members of the House intelligence committee pushed for information on the blocked numbers for more than a year. But House Republicans rebuffed efforts to subpoena the phone records, according to Schiff.

Schiff told MSNBC that he would continue his efforts to “find out who Don Jr. was talking to about that meeting at Trump Tower.”

“Those conversations could have taken place over the phone, or they could have taken place by Don Jr. walking down the hall to talk to his dad,” he added Thursday.