As the impeachment trial of President Trump gets underway, House Intelligence Committee Chairman Adam Schiff, one of seven managers prosecuting the case, said some intelligence agencies have "stopped cooperating" with an ongoing investigation in the House, possibly "on the instruction of others."

"We have requested intelligence, relevant intelligence, concerning Ukraine as a part of our oversight responsibility," Schiff said when asked what evidence his committee is requesting. "The intelligence agencies — some of them have stopped cooperating. And it's our understanding they're doing this on the instructions of others, or with the advice of others."

Schiff said information about Ukraine's reaction to the pressure campaign by Trump allies in 2019 "would be very pertinent" and should be handed over to investigators.

"If there was more evidence that bears on that question and it's being withheld by the intelligence community at the urging of the president, that is a corruption of the intelligence community," Schiff said.

Schiff spoke to "CBS Evening News" anchor Norah O'Donnell, along with three other impeachment managers: Representatives Val Demings, Zoe Lofgren and Jason Crow. The trial is just the third time in U.S. history that a president has faced removal from office.

"CBS Evening News" anchor Norah O'Donnell spoke with four impeachment managers: House Intelligence Committee Chairman Adam Schiff, Representatives Val Demings, Zoe Lofgren and Jason Crow. CBS News

Portions of O'Donnell's conversation with Schiff, Demings, Lofgren and Crow appear below:

Representative Val Demings: I expect not only Leader McConnell, but every member of the Senate, who's taken two oaths, their oath when they were sworn into office and the oath of impartiality, I expect them to take those oaths very seriously and do their job. We want a fair trial.

Representative Zoe Lofgren: When the senators swore impartial justice, they swore to God that they would give impartial justice. And I think that should mean something to them.

O'Donnell: Chairman, is your committee still investigating? And are you hoping to introduce new evidence during this trial?

Schiff: We are still investigating. And new evidence has continued to come to light. And if the senators don't allow witnesses, they'll be the first in history to prohibit them.

O'Donnell: You are now accusing the National Security Agency of withholding critical intelligence that could be useful in this impeachment trial. What specific intelligence or intercepts are you looking for?

Schiff: We have requested intelligence, relevant intelligence concerning Ukraine as a part of our oversight responsibility. The intelligence agencies, some of them have stopped cooperating. And it's our understanding they're doing this on the instructions of others or with the advice of others.

O'Donnell: What is it that you don't feel like you have and that you need?

Schiff: The issue of how Ukraine has responded to the pressure campaign would be very pertinent. If there was more evidence that bears on that question and it's being withheld by the intelligence community at the urging of the president, that is a corruption of the intelligence community.

Demings: The evidence against the president is overwhelming. But the American people deserve to have the complete story.

O'Donnell: Would you acknowledge that even if you were to succeed in calling John Bolton and Mick Mulvaney, that the president could claim executive privilege?

Schiff: that privilege, even if one could be asserted, would have to be asserted narrowly.

Representative Jason Crow: We also have to look-- look at the big picture of all the arguments that the president and his team are making. They're saying that the president can't be indicted. Then they're arguing that the House can't call witnesses and ask for documents. Now they're saying that the president can assert executive privilege over anything that he wants. So if you piece all of those arguments together, it leads to the inevitable conclusion that the president cannot be held accountable and is above the law. And I'm very confident that that's not what the framers intended.

O'Donnell: The White House says you failed to allege any crime or violation of the law. What's the crime?

Schiff: Indeed when the Constitution was written, there really weren't a host of federal crimes. Those came later. What they had in mind was crimes against the body politic. The abuse of power here was the withholding of military aid to an ally at war to help the president cheat in the next election. As one of the Constitutional experts testified, "If that's not impeachable, nothing is."

O'Donnell: The president's legal team says he did not obstruct Congress because he acted with "extraordinary and unprecedented transparency by declassifying and releasing those call summaries."

Lofgren: He has not released a single document. He tried to obstruct every single member of the administration from appearing before the Congress responding to this abuse. This has never before happened in the history of the United States.

Schiff: You really have to give them credit for nerve though, Norah. The argument really boils down to this in their answer. "We have to obstruct the Congress to protect the Congress. We have to subvert the Constitution to save the Constitution. We have to violate our national security to further our national security." Those are absurdist arguments but those are the ones that they're making.

Demings: And let us remember, the release of the call record was not about transparency. The release of the call record was about being caught. And he had no choice but to release the record. And then tried to defend what was indefensible.