House investigators are planning on a round of closed-door interviews with associates of President Donald Trump this summer, promising to bring in a former national security adviser to the campaign, while defending their decision to hear Roger Stone's testimony in private.

Former Trump national security adviser JD Gordon told CNN that members of the House intelligence committee had planned to speak to him Wednesday, but the classified hearing was abruptly canceled because of a scheduling conflict.

Gordon, who was a member of the Trump campaign's national security team, told CNN that the committee staff canceled the meeting this week because of a scheduling issue. He added that he is "perfectly willing to answer any questions."

Gordon has said he had just two brief interactions on the same day with Russian ambassador to the US Sergey Kislyak last year during the week of the Republican National Convention -- and insisted he's done nothing wrong. He was also involved in efforts to draft the GOP platform at last year's RNC. That process has drawn scrutiny because of a dispute among delegates and Trump campaign officials about language regarding US policy on Ukraine. Gordon has said the platform was written in order to align the language with Trump's public comments and positions.

Interviews with other Trump associates, including former campaign chairman Paul Manafort and former policy adviser Carter Page, have yet to be scheduled, congressional sources told CNN. Page told CNN Wednesday he is "ready, willing and waiting" to be interviewed, though he has said he'd like to testify publicly, something the committee is unwilling to do. The committee sent invites to a total of six witnesses last week, according to multiple sources.

Similarly, Stone has repeatedly said that he wanted to testify in public, but the panel refused to let him do so. Now Stone will testify in a classified session July 24.

On Wednesday, Rep. Adam Schiff, the top Democrat on the investigation, defended the panel's decision to host Stone in private, despite Stone's protests to do so publicly.

"We are treating all of what we consider to be the percipient witnesses the same," Schiff told CNN. "(Rep. Mike) Conaway and I have agreed that all of the ... witnesses will be interviewed in closed session."

The witness scheduling continues an aggressive push from the House as it enters a summer set to be dominated by intensive witness questioning.

The committee interviewed former Pentagon official under President Barack Obama, Evelyn Farkas, Monday, and former chairman of the Hillary Clinton campaign, John Podesta, Tuesday.