(NewsNation) — Fentanyl trafficking that officials previously saw in San Diego, California, has changed course, now flowing into ports of entry in Arizona.

Border agents based in San Diego had long accounted for the most fentanyl seizures at the Southwest border. But starting last summer, the Tucson, Arizona, field office has intercepted the most fentanyl, according to an analysis by the Washington Office on Latin Americans.

The reason for the shift is unconfirmed, though experts told NewsNation it’s likely a sign of changing cartel tactics paired with overwhelmed border staff. The U.S. Department of Human Services declined to speculate as to why Arizona might be intercepting more drugs in recent months.

The switch happened between July and August of 2022, according to numbers recorded by U.S. Customs and Border Protection. In July, the majority of fentanyl seizures at the border — 1,122 pounds worth — were conducted by the San Diego Field Office. The Tucson Field Office surpassed that, seizing 1,312 pounds the following month.

Cochise County Sheriff Mark Dannels — whose jurisdiction falls in the Tucson Field Office’s sector — said the location change seems like typical cartel behavior.

“The cartels are a business and they’re a tactical business when it comes to maneuvering their product,” Dannels said.

Independent Arizona Sen. Krysten Sinema is expected to visit the border this week with Dannels. Her office did not reply to NewsNation’s request seeking comment.

Sinema and other members of Congress recently traveled to Mexico City to meet with Mexican government officials, urging for cooperation at the border.

Several factors could be driving the rise in seizures, Dannels said. Those include immigration policy changes that took place between the Trump and Biden administration as well as manpower and overall resources available to the different agencies tasked with managing day-to-day Southern border activity.

According to Dannels, migrants crossing the southern border often tell law enforcement they believe the border is open and that consequences for crossing without authorization have been reduced.

Cubans in particular have been migrating to the U.S. in larger numbers for the past several decades. A December surge in Cuban and Nicaraguan arrivals amounted to the highest number of unauthorized border crossings recorded during Biden’s presidency.

Soon after, Biden announced plans to turn away Cubans, Haitians, Nicaraguans and Venezuelans who showed up at the border without properly filing for asylum.

At the same time, the U.S. has seen dwindling Border Patrol officer numbers since 2016, according to a U.S. Department of Homeland Security’s 2019 report on attrition, the most recent report of its kind available. Factors cited included other jobs, retirement, termination and resignation.

“You have a more porous border right now when it comes to what’s coming across,” Dannels said. “You look at the Tucson sector, like ours, most time we’re leading the nation in got-aways — these are people that are seen are never caught. How do we know what they’re bringing in?”

Law enforcement reports tracking where drugs are entering the U.S. tend to be based on what they see first-hand, said Andrew Arthur, a resident fellow in law and policy at the Center for Immigration Studies.

Arthur previously worked for the Immigration and Naturalization Service as an attorney and acting chief of the INS National Security Law Division.

He believes the cartels are taking advantage of more porous sectors in between ports of entry.

“The Biden administration is attempting to funnel would-be illegal migrants in through those ports of entry,” Arthur said. “The ports were never built for that, and the ports are not staffed for that.”

Cartels will exploit that use of manpower to smuggle drugs where they’re less likely to be caught, Arthur said.

“My conclusion is that the drug cartels are taking advantage of an overwhelmed Border Patrol to move drugs between the ports of entry,” Arthur said.

It’s debated, however, whether more drugs are smuggled through ports of entry or the sectors in between them.

The number of times law enforcement stopped migrants at at southern land borders — both at and between ports of entry — rose 40% from Dec. 2021 to Dec. 2023, according to Customs and Border Protection data.

“The odds of getting caught now compared to years past is less,” Dannels said. “So as long as we decrease our border security, (cartels) are going to increase their wealth, which is their product.”

Since Oct. 2022, 92% of the U.S. border authorities’ fentanyl seizures have occurred at ports of entry. The remaining 8% was seized by Border Patrol agents between ports of entry, according to WOLA.

That’s in line with recent testimony from U.S. Secretary of Homeland Security Alejandro Mayorkas.

“Ninety percent of fentanyl is brought through ports of entry,” Mayorkas said. “Through vehicles, through trucks, through pedestrians.”

Arthur, however, questioned that sentiment, claiming “we’re never going to know the answer.”

“We never know what we don’t catch,” he said.