The tens of thousands of illegal immigrants Border Patrol intercepts each month is a fraction of the overall traffic crossing the U.S.-Mexico border, according to the president of a union representing Border Patrol agents.

“A safe estimate from the Border Patrol agents would be that we arrest about 40 percent of what actually crosses,” National Border Patrol Council President Brandon Judd testified Thursday before a House Judiciary Subcommittee on Immigration and Border Security hearing. He was responding to questions from Rep. Lamar Smith (R-TX) about how many illegal immigrants are able to evade Border Patrol.

“So if you’ve got the official numbers from the agency about what we actually arrest, that’s about 40 percent of what comes,” Judd added.

The Border Patrol union head further recalled a trip House Oversight Committee Chairman Jason Chaffetz (R-UT) took to the U.S.-Mexico border with Border Patrol.

“He had every single resource available to him,” Judd said. “There were seven drug smugglers that crossed the border while he was there that he got to chase. Of those seven drug smugglers that he got to chase with every single one of those resources, they caught zero.”

Judd added that the patrol agent in charge of the station told Chaffetz, “If they hit a sensor, we arrest probably 50 percent. If they don’t hit a sensor, it’s well below fifty percent”

Later in the hearing Rep. Luis Gutiérrez (D-IL) took issue with the description of the illegal crossers as “drug smugglers,” saying Border Patrol could not know for sure if the crossers were smugglers, since they were not caught.