(CNN) Editor's note: This story originally published on May 17, 2018. It reflects information as it was available from documents at the time of publication. For more recent information that came to light in 2019, please see this story .

One of the biggest lingering questions following Wednesday's release of thousands pages of documents related to the June 2016 Trump Tower meeting is three phone calls Donald Trump Jr. placed to blocked telephone numbers before and after the meeting.

Trump Jr.'s calls to blocked numbers — one of which occurred between phone calls with Emin Agalarov, the pop star son of Russian oligarch Aras Agalarov — have raised questions about who President Donald Trump's eldest son spoke to. Democrats have suggested Trump Jr.'s calls could have been to his father, but Trump Jr. said he did not know.

"So you don't know whether or not this might have been your father?" congressional investigators asked Trump Jr. during his interview, according to the transcript of the interview released by the Senate Judiciary Committee.

"I don't," Trump Jr. responded.

CNN has reached out to Trump Jr.'s attorney for comment and have not yet received a response.

The phone calls to blocked numbers are one of the key unanswered questions that Democrats — on both the Senate Judiciary and House Intelligence Committees — charge that Republican investigators failed to follow up on with subpoenas to Trump Jr. It's one of the items that could receive renewed scrutiny in the committees should Democrats win back either chamber in November.

Trump Jr. has said he did not speak to his father about the Trump Tower meeting, in which he was expecting "dirt" on Hillary Clinton from a Russian lawyer, but instead received a pitch on removing Russian sanctions under the Magnitsky Act.

The Judiciary Committee Democratic report released Wednesday highlighted the blocked calls, the first of which was a four-minute call June 6 less than an hour after Emin Agalarov, who had pushed for the Trump Tower meeting, had called Trump Jr. After calling the blocked number, Trump Jr. called back Agalarov. He told the committee that he did not believe he spoke with Emin Agalarov in either phone call, but that voice messages may have been exchanged.

Trump Jr. also called a blocked number that evening in a call that lasted 11 minutes.

Then-candidate Trump spent that day at Trump Tower, and had no public events.

The Judiciary Committee Democrats noted that while Trump Jr. said he didn't know who he called, Trump campaign manager Corey Lewandowski had testified to the House Intelligence Committee that Trump's "primary residence has a blocked (phone) line."

Roughly two hours after the Trump Tower meeting occurred on June 9, Trump Jr. placed another call to a blocked number that lasted three minutes, according to the Democratic report. It doesn't appear Trump Jr. was asked about that specific call during the interview.

The Judiciary Committee Democrats also highlighted in their report an announcement from Trump ahead of the Trump Tower meeting that he would be giving a "major speech" the following week, in which "we're going to be discussing all of the things that have taken place with the Clintons. I think you're going to find it very informative and very, very interesting."

Democrats have urged Senate Judiciary Chairman Chuck Grassley, an Iowa Republican, to bring Trump Jr. back for a follow up interview, as well as to subpoena his phone records to find out who he called.

"There is a lot that needs to be explained, the contradictions and evasions," Sen. Richard Blumenthal, a Connecticut Democrat, said Wednesday.

Asked about a subpoena for Trump Jr.'s records, Grassley said he put out the transcripts in the interest of transparency, and added that he would want to know what questions Democrats would want to ask if he were to consider bringing him back.

"I have no decision now," Grassley said. "In fact, it's just come to my attention, not only through your question but a few minutes ago."