Border Patrol observed but was unable to arrest more than 600,000 immigrants who illegally entered the United States over the past year, in addition to the 2 million people that agents were able to apprehend and arrest, according to a Biden administration official.

Department of Homeland Security Secretary Alejandro Mayorkas testified before the Senate Tuesday that hundreds of thousands of immigrants who illegally crossed the border from Mexico or Canada in fiscal 2023 got away without being encountered, screened, or vetted.

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"That number is over 600,000," Mayorkas told Sen. Roger Marshall (R-KS) during a Senate Homeland Security and Governmental Affairs Committee hearing.

U.S. Border Patrol agents and cameras on the border observed and tracked 600,000 unidentified people who slipped into the country between October 2022 and September 2023, but overworked agents were unable to track down and apprehend everyone.

Marshall asked Mayorkas rhetorically how many of the 600,000 known "gotaways" had convictions for rape or murder in their countries of origin, as well as how many had been previously deported from the United States.

Mayorkas defended the DHS's handling of the border and said the U.S. was experiencing an "unprecedented" number of displaced people across the Western Hemisphere and that the country's border crisis was not helped by the "broken immigration system" that Congress has not addressed.

Marshall asked if any of the 600,000 "gotaways" posed a risk to Americans.

Mayorkas said illegal immigrants who meet certain criteria — terrorism affiliation, espionage, and select aggravated felons — would be arrested and face deportation under guidelines rolled out under the Biden administration that limited which illegal immigrants could face deportation.

"This is why I believe the defining issue in this next election is going to be exactly this, 'Are our families safer today than they were three years ago?'" Marshall said.

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Marshall also inquired about the countries of origin of the 169 known and suspected terrorists who Border Patrol agents arrested at the U.S.-Mexico border in 2023.

Mayorkas declined to provide the information publicly and said he was available to speak about it in a private setting.