Customs and Border Protection reports its agents apprehended about 50,000 people in March and a similar number in April.
Those numbers are up more than 200 percent from the prior year.
At the same time, the number of Border Patrol agents in the Tucson Sector alone has dropped from 4,400 to 3,700, according to an agency spokesman.
"Recently there has been a spike in the number of illegal crossings," Agent Chris Sullivan said. "It's definitely something we have to address and look at."
Apprehensions along the southwest border can fluctuate significantly, and 2017 was an extremely light year.
Even with a sudden spike this spring, apprehensions are far below peak immigration years in 2005 and 2006, which averaged more than a million people apprehended in a year.
In recent years, criminal and gang member captures have also declined.
"When we see our goal, you know it's having zero [unauthorized] people crossing the border," Sullivan said.
"It's sad that there are these type of illegal activities going on, but it's fantastic that I'm able to participate in assisting these agents," Sgt. Scott Engels, a Guard member from the Phoenix area, said.
It's unclear how long the National Guard could be stationed at the border, but funding for the mission is guaranteed until the end of September.