As the investigation into possible collusion between the Trump campaign and Russia deepens, Sen. Kamala Harris offered a crucial declaration: Donald Trump himself may find himself under questioning.

As special counsel Robert Mueller’s inquiry into possible collusion between the Trump campaign and Russia reaches the highest levels of federal government, one senator made clear that Donald Trump should be on notice.

California Democratic Sen. Kamala Harris told MSNBC’s Ari Melber that the Senate may well seek Trump’s testimony as the investigations continue.

Mueller’s investigation grows deeper and wider seemingly every day. Following testimony by Donald Trump Jr. about his meeting with Russian operatives, the probe expanded with interview requests for any White House staff members who were involved in crafting the statement, later proven to be false, about that meeting.

Trump himself was notably involved in creating that mendacious statement. And Harris declared that his presence at a hearing is a very real possibility.

Harris, a former prosector and former attorney general of California, noted the importance of looking not just backward at what already happened, but forward as well, to “understand that what happened can happen again.”

When Melber asked if she thinks the Trump administration is doing anything to prevent another election hack in 2018, Harris could only reply, “I don’t know. I don’t know.”

But with a lack of acknowledgment from the executive branch that something of this magnitude truly occurred, “It’s hard for me to believe that you’ll understand that you need to then avoid the thing that happened in the past from happening again.”

Melber mentioned the private testimony with Trump Jr., as well as others, and asked if potential testimony from Donald Trump is “off the table.”

“No,” Harris replied succinctly.

HARRIS: We cannot only look, as I would as a prosector, at what happened, right? Look in the rearview mirror, what happened, so we can attribute accountability and also consequence. We also must look forward, and understand that what happened can happen again. And for that reason there should be a sense of urgency that we all apply to the facts, and getting to the truth of what happened.

MELBER: To that point, do you think the Trump administration is stopping Vladimir Putin from doing this in 2018?

HARRIS: I don’t know. I don’t know. But I’ll say this: If you don’t acknowledge something everyone has acknowledged, which is that there was that interference, it’s hard for me to believe that you’ll understand that you need to then avoid the thing that happened in the past from happening again.

MELBER: There is a private interview today with the president’s son, Don Jr. There are other interviews and hearings. Is the testimony or potential hearing with the president, Donald Trump, off the table for this investigation?

HARRIS: No.

MELBER: So it’s possible that the Senate could seek his testimony?

HARRIS: I think it’s possible, yes.

MELBER: Under what conditions?

HARRIS: We’ll see.

MELBER: “We’ll see” — you sound a little bit like Donald Trump.

HARRIS: Oh, not at all! [laughter]

It has been clear to anyone paying attention that Trump has been involved in various ways throughout this entire debacle, sending his lawyers out to lie repeatedly for him and musing on live television about the fact that he fired FBI Director James Comey because of the Russia investigation, as well as helping with the cover-up of his son’s meeting with Russian agents.

Trump was the name at the top of the campaign ticket. He was running that show. Which means any machinations or deceit that occurred did so under his watch — and potentially at his direction.

The American people deserve to know the absolute truth of this matter, and that means hearing directly from the man in charge of it. And Harris is ready and willing to make that happen.