(CNSNews.com) - Dozens of classified Defense Department documents were leaked to the media last month, revealing details of U.S. surveillance of Russia’s military operations against Ukraine, an assessment of Ukraine’s military, and intelligence on U.S. allies like South Korea and Israel.

President Biden has been briefed on the leaks last week “when we all got word that there were some documents out there, and he has been-- stayed briefed and in contact with national security officials throughout the weekend,” NSA spokesman John Kirby said Monday.

“So he has been briefed, and as for the source, as you know, the Department of Defense has referred this to the Department of Justice for a criminal investigation, and I certainly would refer you to them, but I’m not aware that they’ve come to any conclusions at this point about where they’re coming from,” he said when asked whether the administration has any sense of who was behind the leaks.

When asked whether the administration believes the leak is contained and whether there are more documents out there that haven’t been released, Kirby said, “We don’t know. We truly don’t know.”

CBS News Senior White House Correspondent Weijia Jang asked, “In light of the classified documents, I wonder if you could tell us when was the last time the protocol to grant security clearances was updated or revised in the government, and in light of the leaks, does the president believe he should take another look at that?”

“I think the Department of Defense has already started to take a look at distribution for instance at the Pentagon and let them speak to that. I don’t know the answer to your question on protocols for who is and who is not granted security clearances,” Kirby said.

“There is already in place and remains in place a very diligent, deliberate effort to manage security clearances, and sometimes we get knocked, because it takes a little too long for people’s clearances to get approved because the process is so careful and deliberate,” he said.

“I think to your question, I think we just need to be careful right now speculating or guessing what might be behind or who might be behind what looks like a potential leak here of classified information, so we’re gonna– we need to let the process sort of bear itself out,” the NSA spokesman said.

“Are you saying there’s a question of whether this was a leak by someone who had access to the materials?” Jang asked.

“I’m saying we don’t know, which is why I’m gonna– I think you’re getting to remedies, and I understand that. We’re only a couple of days into this. We need to let the Department of Justice do their job, investigate this, see what they learn,” Kirby said. 

“Now again, that said, the Department of Defense has already said that they’re taking a look at distribution and looking if there’s changes they need to make administratively there at the Pentagon, since it appears that they are, they have the locus of most of the documents, but again before we start making major muscle movements, we need to know what we’ve got here and have a better sense from the Justice Department,” he said.