An estimated 20 million people tuned in to live U.S. television broadcasts of Thursday's hearing by lawmakers probing the 2021 riot at the U.S. Capitol, the Nielsen ratings agency said on Friday.

The viewership across 12 networks ranked below other political events such as President Biden's State of the Union address, which pulled in 38 million viewers in March, but higher than most congressional hearings.

The first televised hearing of the impeachment inquiry into U.S. President Donald Trump, for example, attracted about 14 million viewers on 10 networks in 2019. It was held during the day, when TV viewership is lower.

The committee probing the Jan. 6, 2021, attack by Trump supporters is holding rare primetime hearings to spotlight the findings of its nearly year-long investigation. Broadcasters including ABC, CBS and NBC interrupted regular programming to show the event.

Nielsen's figures included people who watched via traditional television or streamed through Internet-connected TVs, but they do not capture the full extent of online viewing via social media.

The tally also does not include Fox News Channel, which opted to run its regular opinion programming during the hearing. Fox Business Network covered the hearing live.

Of the 20 million viewers, 15.2 million were age 55 and older, Nielsen said. Less than 1 million people age 18 to 34 watched via the TV networks.