FIRST ON FOX: Rep. Andy Biggs, R-Ariz., is warning that Border Patrol may be undercounting the number of illegal immigrant "gotaways" at the southern border as the U.S. continues to face a crushing migrant crisis.

Biggs, the co-chair of the House Border Security Caucus, said in a letter to Homeland Security Secretary Alejandro Mayorkas, obtained first by Fox News Digital, that while official statistics indicate that there have been around 675,000 "gotaways" this fiscal year, the number could be higher.

The term "gotaways" refers to illegal immigrants who evade Border Patrol but who are detected on another form of surveillance. Biggs warned that under current practice, agents report daily on the numbers they believe got by them, but this can be inaccurately reported.

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Rep. Andy Biggs is pushing for answers about the number of gotaways seen at the border. (Chip Somodevilla / Getty Images / File)

"However, when agents report groups of aliens that exceed 20, no matter the number above, the group is reported as ‘20+’ and counted as 20," he wrote.

"For years, sources have claimed this maximum category of 20+ has led to underreporting. Now, given the sheer volume of people entering the United States illegally, gotaway groups regularly exceed that number, leading to further underreporting," Biggs continued.

He warned that gotaways pose a significant threat to U.S. communities and national security and are more likely to have criminal backgrounds or cartel affiliations – both of which pose a threat that officials have acknowledged.

"Given their evasion of law enforcement, these individuals likely have criminal backgrounds or cartel affiliations that would deem them inadmissible for legal entry or are trafficking illegal drugs, weapons, or people into the country," he said.

Biggs is seeking information on how DHS counts gotaways, its estimates for fiscal year 2023 and changes it intends to make to ensure better reporting of the number.

A Customs and Border Protection (CBP) spokesperson, however, told Fox that Border Patrol does count for migrant groups above 20 "based on the totality of migrants that it can confirm" and that there is no pre-set limit to that data. The spokesperson said that agents count and record "all migrants it identifies as the accurate number of gotaways." 

"In cases where agents cannot determine with certainty the total number of gotaways by tracking footprints, the number of gotaways recorded is the number of footprints confirmed, as well as those verified through a multitude of assets including cameras along the border," the spokesperson said. 

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DHS has previously pointed to moves it has made to increase in technology and personnel at the border, and officials have said that agents actually have gained more awareness of what is going on at the border than before. It has also touted what it says is a high level of apprehensions – with over 70% of crossings resulting in an apprehension in 2021, remaining the same as the prior administration. 

"With the investments that this Congress has made into the Border Patrol, CBP as a whole, we have greater situational awareness now than I’ve ever had," then-Border Patrol Chief Raul Ortiz said in March. Current Border Patrol Jason Owens has made similar remarks about how the agency has increased detection capability.  

The letter comes as numbers have again skyrocketed at the southern border, including a surge of thousands into Eagle Pass, Texas, this week. Sources told Fox on Thursday that there were more than 10,000 encounters across the border in a 24-hour period. Multiple Border Patrol sectors have been left overwhelmed, with some sectors resorting to releasing detained migrants to the streets.

Sources have also said that numbers for August appear to be on track to be around 230,000, a significant increase from the more than 180,000 encountered in July and 144,000 encountered in June.

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The administration announced a slew of measures this week to deal with the crisis, including additional deportation protections for hundreds of thousands of Venezuelans and increases in Border Patrol facility capacity, expedited work permits and personnel deployments to the border. It also renewed calls for more funding from Congress, as well as the passage of an immigration reform bill to fix what it says is a "broken" immigration system. 

Fox News’ Bill Melugin contributed to this report.