A tragic reminder came this week when at least 51 people died after being abandoned in the back of a tractor-trailer in sweltering San Antonio. Authorities believe the vehicle was part of a human-smuggling operation.

While the scale of the calamity was shocking, it’s only the most recent example to illustrate how U.S. officials have struggled to find the right strategy for patrolling the border and preventing deaths.

Lax enforcement can encourage more people to travel north in hopes of a better life. But clamping down is not always a deterrent. Instead, migrants may rely on riskier routes to avoid detection, or put themselves in the hands of smugglers who promise that they can evade authorities for a price.

The San Antonio tragedy triggered familiar reactions across the U.S. political spectrum, indicating that a solemn record as the deadliest smuggling attempt in the nation’s history will do little or nothing to reshape a debate that has hamstrung Washington for decades. Finger-pointing began almost immediately.

President Joe Biden, in Europe this week for international summits, said the deaths were “horrifying and heartbreaking.”

“Exploiting vulnerable individuals for profit is shameful, as is political grandstanding around tragedy, and my administration will continue to do everything possible to stop human smugglers and traffickers from taking advantage of people who are seeking to enter the United States between ports of entry,” Biden said.

The migrants were discovered on Monday when a city worker heard a cry for help from the abandoned truck that was parked on the side of a back road. Dozens were already dead; more died at nearby hospitals.

Texas Gov. Greg Abbott, a Republican who is investing billions of dollars of state money on border security, tweeted within hours of the grisly discovery that the deaths were “on Biden.”

“They are a result of his deadly open border policies. They show the deadly consequences of his refusal to enforce the law,” Abbott said.

Immigration advocates disagreed with Abbott’s criticism and said Biden was too focused on enforcement. A federal judge has kept in place a Trump-era policy that denies many migrants a chance to seek asylum on grounds of preventing spread of COVID-19.

“If the Biden administration continues to illegally turn away migrants and deny their chance to rightfully seek asylum, individuals and families escaping persecution, war, and climate disasters will continue to face violence and death,” advocacy group RAICES Texas said in a statement.

White House press secretary Karine Jean-Pierre told reporters aboard Air Force One — as Biden was flying between summits in Germany and Spain — that the administration was focused on the victims and holding human smugglers accountable.

“The fact of the matter is, the border is closed, which is in part why you see people trying to make this dangerous journey using smuggling networks,” she said.

The U.N. migration agency has reported that nearly 3,000 people went missing or died trying to cross the border from Mexico into the United States since 2014. The San Antonio tragedy pushed the total to nearly 300 for the first half of this year.

The International Organization for Migration, along with the U.N. refugee agency, called for a swift investigation.

“Without sufficient pathways to safety, vulnerable and desperate people will continue to be preyed upon by smugglers or forced to resort to desperate measures to cross borders,” said Matthew Reynolds, the U.N. High Commissioner for Refugees’ representative to the United States and Caribbean.

U.S. Customs and Border Protection, which counts deaths differently, reported 557 people perished on the southwest border in the 12 months ending Sept. 30, more than double the 247 deaths reported in the previous year and the highest since it began keeping track in 1998.

Deaths became commonplace on the border after “Operation Gatekeeper,” launched in 1994, pushed migrant traffic to the Arizona deserts from San Diego. Despite billions of dollars spent every year on border security since then, neither Republican or Democratic administrations have been able to stem the loss of life.

Migrants routinely take risks to cross into the United States.

Jose Castillo, 43, left Nicaragua with his wife and 14-year-old son in January but didn’t cross the Rio Grande in Eagle Pass, Texas, until May, paralyzed with fear that they would drown. He and his wife finally decided that one of them could die, as long as their child made it safely. They took a chance — and it worked.

“We can never return to Nicaragua,” he said.

Under Trump and Biden, Border Patrol agents have been stretched extremely thin because they spend long stretches of time processing cases for immigration court. Such responsibilities take them out of the field, making it easier for people to cross undetected. The Border Patrol recently began releasing tens of thousands of migrants on parole in hopes of freeing up more agents to be in the field to try to stop migrants.

The number of people found crossing the border illegally is at or near the highest in about two decades. Decisions to migrate are complex, but it could be that many people are getting through undetected and encouraging others to come. Migrants who succeed sometimes tell their stories to family and friends back home, encouraging them to follow.

At the same time, Title 42 has encouraged repeat attempts to cross the border because there are no legal consequences, such as criminal charges or records of deportation, for getting caught. Many people cross several times until they succeed.

It’s unclear whether any of the migrants who died in San Antonio had previously been expelled.

Isis Peña, 45, fled Honduras with a friend, who urged her to cross the border illegally. Peña refused but began to regret her decision after the friend soon called from San Antonio to say she made it easily and U.S. authorities didn’t even ask her any questions before getting released.

The next day, Peña tried to cross. Although she made it across the river, she was expelled to Mexico under Title 42 authority.