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WikiLeaks founder Julian Assange doubled down Thursday on his accusation that the CIA had "lost control" of its arsenal of cyber weapons, as investigators hunted for those responsible for a leak that experts said already is doing major damage to U.S. intelligence gathering.

Assange offered no proof of his explosive and expansive claim, which, if true, would amount to a body blow to America's efforts against terrorists and foreign adversaries. And computer researchers say they saw no sign of CIA cyber exploits for sale in the darker corners of the internet.

Still, it was becoming clear that the disclosure Tuesday by WikiLeaks of nearly 9,000 documents describing the CIA's cyber tools and methods was a serious setback for American spying, in and of itself, multiple current and former U.S. officials told NBC News.

"This is bad,” said a senior U.S. official briefed on the damage. "It’s going to make it harder to collect on terrorists, on the Russians, on the Chinese — you name it."

Former CIA Director Leon Panetta told NBC News he believed the leaks were already having an impact.

"Information in the hands of our enemy in the hands of others undermines the ability of the United States to gather the intelligence necessary in order to protect our country," he said. "And so what they are doing is literally weakening the security of the United States.

"CIA is going to have to devote itself now to looking at new tools to be able to get the information necessary from potential terrorists who are out there in order to ensure that we continue to provide good intelligence to the president and to others in order to protect our country."

The leaked documents don’t include the cyber weapons themselves, but they hint that the CIA has been able to exploit vulnerabilities in mobile phones, smart TVs and other devices as it gathers intelligence overseas. They also reveal secrets about CIA tactics, techniques and locations.

President Donald Trump’s spokesman, Sean Spicer, said the president had "grave concern" about national security information being released, and "obviously believes the systems at CIA are outdated and need to be updated."

Assange, meanwhile, taunted the CIA, accusing the agency of "a historic act of devastating incompetence to have created such an arsenal and stored it all in one place and not secured it." Assange raised the specter that some CIA hacking tools were now in the possession of foreign governments or criminals.

WikiLeaks says the leaked documents were shared among CIA employees and contractors using a collaboration software called Confluence, made by an Australian firm called Atlassian. Confluence allows thousands of people to work on the same files.

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In a statement to NBC News, Atlassian spokesman Pail Loeffler said, "We're not able to comment on if CIA is a customer. We take the privacy and security of our customers and their data very seriously. We currently do not have any indication that our systems or networks have been breached."

Related: WikiLeaks Mystery: How Were CIA Hacking Tools Stolen?

Assange spoke from the Ecuadorian embassy in London, where he lives to avoid extradition over sexual assault charges in Sweden.

No one from the U.S. government explicitly denied Assange’s sweeping claim, but outside experts were skeptical. Some wondered how Assange could possibly know how many hacking tools the CIA possessed, even if a large number were compromised.

"We have never seen the authentic tools from Vault 7 leak on the 'black market'," computer researcher Andrew Komarov told NBC News, using the name WikiLeaks gave to the leaked document archive.

The CIA meanwhile, pushed back in general terms.

"Julian Assange is not exactly a bastion of truth and integrity," CIA spokesman Dean Boyd said. "Despite the efforts of Assange and his ilk, CIA continues to aggressively collect foreign intelligence overseas to protect America from terrorists, hostile nation states and other adversaries."

Assange said WikiLeaks had purposely not released the actual cyber weapons in its possession, to prevent them from falling into the wrong hands. He said he would cooperate with tech companies to neutralize the exploits before releasing them.

"We have decided to work with them to give them exclusive access to the additional technical details we have," he said.

Related: WikiLeaks Releases Alleged CIA Hacking Secrets

But current and former U.S. officials cast aspersions on WikiLeaks, which the U.S. government has branded as a collaborator with Russian intelligence agencies.

The leak "does grave damage to the CIA on several levels," said Juan Zarate, a former U.S. intelligence official and an NBC News consultant. "It hurts CIA’s credibility and morale, and exposes capabilities that the CIA has invested in likely millions of dollars and multiple man hours."

Wikileaks' claim that the leak came from a U.S. contractor has focused scrutiny on the extent to which the CIA relies on contract employees, particularly in its cyber operations.

Contractors don't always undergo the same grueling background checks that employees do, former CIA director Leon Panetta told NBC News.

"You do take a chance with contractors," Panetta said. "They don’t work on the inside. They may not have the same loyalty to the organization."

.@SenFeinstein on latest wikileaks: We have a lot of contractors working in these agencies... should cause us concern. #AMR @MSNBC — Andrea Mitchell (@mitchellreports) March 9, 2017

Some in Congress say relying on contractors is too risky.

"I'm one that believes that government employees should be doing this and not contractors," Sen. Dianne Feinstein, Democrat of California, told NBC News.

The CIA found out about a breach of its hacking tools late last year, U.S. officials told NBC News. Assange wondered whether the agency warned anyone at that time.

"Why has the Central Intelligence Agency not acted with speed to come together with Apple, Microsoft and other manufacturers to defend us all from its own weapons systems," he asked.

Meanwhile, U.S. Senator Ben Sasse, R-Nebraska, urged the Trump administration to prosecute Assange.

Related: Julian Assange Says WikiLeaks May Release More CIA Hacking Tools

Sasse, a member of the Senate Armed Services Committee, called Assange "an enemy of the American people and an ally to Vladimir Putin" who "has dedicated his life's work to endangering innocent lives, abetting despots, and stoking a crisis of confidence in the West."