GEOFF BENNETT: The country's top intelligence officials testified in the Senate today, assessing Russia's plans in Ukraine, the threat of TikTok and the origins of COVID, which, as Nick Schifrin reports, was also the subject of its own hearing in the House.

SEN. MARK WARNER (D-VA): Good morning.

I'm going to call this hearing to order.

NICK SCHIFRIN: In the Senate Intelligence Committee today, the woman and men who lead the country's intelligence community detailed a world full of threats, starting with Russia's war in Ukraine.

Wagner private military contractors are besieging the eastern city of Bakhmut, the longest and deadliest battle of the war.

Ukraine is trying to inflict heavy Russian losses.

Those losses are adding up, Director of National Intelligence Avril Haines said today.

AVRIL HAINES, U.S. Director of National Intelligence: It will be increasingly challenging for them to sustain even the current level of offensive operations in the coming months.

And, consequently, they may fully shift to holding and defending the territories they now occupy.

NICK SCHIFRIN: Haines said Russian President Vladimir Putin is focused on -- quote -- "more modest" military objectives, but warned he could fight a long war.

AVRIL HAINES: Putin most likely calculates that time works in his favor, and that prolonging the war, including with potential pauses in the fighting, may be his best remaining pathway to eventually securing Russia's strategic interests in Ukraine, even if it takes years.

SEN. MARCO RUBIO (R-FL): Could they use TikTok to control data on millions of users?

NICK SCHIFRIN: The hearing also focused on technology companies that answer to the Chinese Communist Party, or CCP, especially the hugely popular video app TikTok.

A new bipartisan bill could lead to a complete ban, and is supported by the White House.

Today, Intelligence Committee Vice Chairman Marco Rubio asked FBI Director Chris Wray about the app's potential threat.

SEN. MARCO RUBIO: Could they use it to control the software on millions of devices given the opportunity to do so?

CHRISTOPHER WRAY, FBI Director: Yes.

SEN. MARCO RUBIO: Could they use it to drive narratives, like to divide Americans against each other?

CHRISTOPHER WRAY: Yes.

This is a tool that is ultimately within the control of the Chinese government.

And it -- to me, it screams out with national security concerns.

SEN. KIRSTEN GILLIBRAND (D-NY): The fact that the intelligence community still disagrees on the origins of COVID is concerning.

NICK SCHIFRIN: Senators also asked about the origins of COVID-19, which began spreading at the end of 2019 in the central Chinese city of Wuhan.

CHRISTOPHER WRAY: The FBI has long assessed going all the way back to the summer of 2021 that the origin of the pandemic was likely a lab incident in Wuhan.

NICK SCHIFRIN: But the intelligence community remains divided.

The FBI concluded with moderate confidence that COVID leaked from a Wuhan lab, an assessment now shared by the Department of Energy with low confidence.

But other agencies have assessed, also with low confidence, COVID likely occurred naturally.

AVRIL HAINES: China has not fully cooperated.

And we think that is a key, critical gap that would help us to understand what exactly happened.

REP. BRAD WENSTRUP (R-OH): Where did COVID-19 come from?

NICK SCHIFRIN: It was also the subject of another, more politicized hearing by the House Select Subcommittee on the Coronavirus Pandemic.

Republicans and their witnesses went beyond the intelligence community conclusions and argued the Wuhan Institute of Virology, with U.S. funding, artificially combined viruses in a process known as gain of function, and those experiments created viruses that mirrored COVID-19's unique attributes.

Robert Redfield is a virologist and was the director of the Centers for Disease Control when COVID began.

DR. ROBERT REDFIELD, Former Director, Centers for Disease Control and Prevention: While many believe that gain of function research is critical to get ahead of viruses by developing vaccines, in this case, I believe it was the exact opposite, unleashing a new virus of the world without any means of stopping it, and resulting in the deaths of millions of people.

REP. DEBBIE LESKO (R-AZ): Do you believe there was a cover-up?

NICK SCHIFRIN: Republicans repeatedly accused Dr. Anthony Fauci, the former head of National Institute of Allergy and Infectious Diseases, of promoting the naturally occurring theory and silencing the lab leak theory.

DR. ROBERT REDFIELD: I don't think I used the word cover-up, OK?

I think there was an attempt to misguide, redirect the debate.

NICK SCHIFRIN: Last night, in The New York Times, Dr. Fauci called Republican allegations false and misleading, and said he's always kept an open mind on COVID's origins.

China has largely stonewalled efforts to investigate independently.

Jamie Metzl is an Atlantic Council senior fellow.

JAMIE METZL, The Atlantic Council: And if we make it primarily about Dr. Fauci, we will be inappropriately serving the Chinese government a propaganda coup.

NICK SCHIFRIN: Most leading scientists still argue COVID spread naturally from humans to animals.

But, regardless of the cause, finding the origin is still critical.

Paul Auwaerter was the president of the Infectious Diseases Society of America.

DR. PAUL AUWAERTER, Former President, Infectious Diseases Society of America: We can learn valuable lessons from these investigations to prevent outbreaks and pandemic's of any origin.

NICK SCHIFRIN: But it's not clear if the world will ever know the origin of a disease that has killed nearly seven million people and counting.

For the "PBS NewsHour," I'm Nick Schifrin.