(CNN)Chris Van Gorder says he's seeing a telling trend in the hospitals he runs.
Coronavirus patients are showing up in emergency rooms after calling 911 from the US-Mexico border.
"They'll literally come to the border and call an ambulance," says Van Gorder, president and CEO of Scripps Health, a hospital system in southern California.
The rise in ambulance traffic from the border, which several officials described to CNN, is a symptom of the pandemic's spread in the region -- and a sign of the many connections between communities in both countries.
"There just is not a wall for viruses at the border," says Josiah Heyman, director of the Center for Inter-American and Border Studies at the University of Texas at El Paso. "The wall is an illusion, because the two sides are really woven together."
An increase in cross-border coronavirus cases, which began getting public attention in May, overwhelmed some California hospitals and spurred the state to create a new patient transfer system to help.
"It's an unprecedented surge across the border," says Carmela Coyle, president and CEO of the California Hospital Association.
In the past five weeks, more than 500 patients have been transferred to hospitals across the state from California's Imperial County, which has the state's highest per capita rate of coronavirus cases -- and, according to officials, has seen a large number of patients crossing from Mexico.
But Van Gorder, Coyle and other officials in California say this isn't an immigration issue.
Most of the coronavirus patients crossing the border, they say, are Americans.
In a call with state hospital leaders earlier this month, the head of California's emergency medical services authority, Dr. David Duncan, described the steady stream of patients coming to Imperial County as "gas on the fire."
"We've got this continual flow of Covid coming across the border in the form of US citizens that carry and continue to escalate and fuel the Covid pressures that we see," Duncan said.
The view from the border
Officials estimate about a quarter of a million US citizens live across the border in the Mexican state of Baja California. Many work in the US and have family members there. Some regularly go to US hospitals when they need medical attention. Others decided to cross this time because Mexican hospitals were overwhelmed by a crush of coronavirus cases.
"What has happened as the situation has worsened on the Mexican side of the line is that a number of the US citizens are returning to the United States to seek care for Covid-19," says Coyle of the California Hospital Association.
Officials in Mexico's Baja California state say the number of active coronavirus cases there are now declining. But the state -- which is home to large cities like Tijuana and Mexicali -- has been hit hard by the pandemic. With more than 1,700 dead, it's among the Mexican states with the highest Covid-19 death tolls and death rates per capita.
Sergio A. Beltrán, US Customs and Border Protection's officer in charge for the Calexico ports of entry, said in a statement to CNN that he started to see an increasing flow of people coming across the border for medical care a few months ago.
"While it varies from day to day, and shift to shift, we have experienced a significant increase in medical-related calls at the Calexico ports of entry that can be directly linked to the COVID-19 pandemic over the last couple of months," he said. "We're definitely seeing people on a daily basis. And we still have our regular medical-related calls that aren't COVID-related from people in accidents or having other medical issues and are coming for medical treatment in the US."
Sometimes people walk to the port of entry or drive themselves to the border crossing, then call for an ambulance to get them to a US medical facility when they arrive. And sometimes, he said, travelers arrive at the border in Mexican ambulances and have already arranged for US ambulances to meet them there.
What happened in California hospitals as more coronavirus patients crossed
At El Centro Regional Medical Center in Imperial County, coronavirus cases started to climb in early May as the situation across the border in Mexicali worsened, says Adolphe Edward, the hospital's CEO.
"That uptick just took a fast, escalating move," Edward says.
At one point that month, the 161-bed hospital saw so many coronavirus cases that it had t