The investigation is being conducted by Justice Department prosecutors in Washington and in the Eastern District of Virginia, along with the FBI, the officials said.

The DOJ and FBI declined to comment.

In response, a ByteDance spokesperson told NBC, "We have strongly condemned the actions of the individuals found to have been involved, and they are no longer employed at ByteDance. Our internal investigation is still ongoing, and we will cooperate with any official investigations when brought to us."

News about the probe was first reported on Thursday by Forbes, which said the employees implicated in the surveillance were fired after it was confirmed they were trying to find the sources of leaks to journalists from inside the company. A ByteDance investigation found that the employees gained access to the IP addresses and other data of the reporters and some of their contacts connected to them through their TikTok accounts, Forbes reported.

The federal investigation into ByteDance comes as concern over the security threats to Americans related to TikTok has intensified this week. The Biden administration has threatened a potential ban on the popular social media app in the U.S. if its Chinese owners refuse to sell their stakes in it, a source close to the company told NBC News on Thursday. That came after U.S. intelligence officials expressed concerns that China can use TikTok to spy on Americans or influence U.S. public opinion.

Asked what message the White House is sending to TikTok users in the U.S., press secretary Karine Jean-Pierre told reporters on Thursday, "We’ve expressed concerns over China’s potential use of software platforms that could endanger or threaten America’s safety and their national security. So that is the president’s concern. That is why we have called on Congress to take action."

Jean-Pierre reiterated that a group of federal agencies are currently conducting a review of TikTok's software.

Biden backs legislation introduced earlier this month by a bipartisan group of senators that would allow the federal government to regulate and ban foreign-produced technology, including TikTok.

The bill would give the secretary of commerce the power to regulate tech produced by six countries that have adversarial relationships with the U.S.: China, Cuba, Iran, North Korea, Russia and Venezuela.

On Capitol Hill, there is an appetite to address the issue, although it's not clear how far lawmakers are willing to go.

Asked Friday if he believes TikTok represents a national security risk, Speaker Kevin McCarthy said, “Yes.”

**Ken Dilanian is the justice and intelligence correspondent for NBC News, based in Washington.

***Rebecca Shabad is a politics reporter for NBC News based in Washington.