WASHINGTON (AP) — President Joe Biden said Tuesday that the deaths of migrants who were in the back of a tractor-trailer in Texas was "horrifying and heartbreaking."

"While we are still learning all the facts about what happened and the Department of Homeland Security has the lead for the investigation, initial reports are that this tragedy was caused by smugglers or human traffickers who have no regard for the lives they endanger and exploit to make a profit," he said in a statement shortly after arriving in Spain on the second stop of a trip in Europe.

Biden added, "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."

At least 50 people died after being left in the back of a tractor-trailer in San Antonio, and authorities said they were likely being transported as part of a smuggling operation. They were found on Monday, when temperatures in the area approached 100 degrees Fahrenheit (38 degrees Celsius) on Monday.

The tragedy, among the worst suffered by people being snuck across national borders around the world, swiftly became a vehicle for political attacks on the Biden administration.

"These deaths are on Biden," tweeted Texas Gov. Greg Abbott, a Republican, within hours of the grisly discovery. "They are a result of his deadly open border policies. They show the deadly consequences of his refusal to enforce the law."

Many of former President Donald Trump's strict border policies — including Title 42, which prevents many migrants from seeking asylum during the public health emergency caused by the coronavirus — have remained in place under Biden, who is a Democrat.

Aaron Reichlin-Melnick, policy director at the American Immigration Council, suggested it was rigid, not lax, enforcement that contributed to the tragedy.

"With the border shut as tightly as it is today for migrants from Mexico, Guatemala, Honduras and El Salvador, people have been pushed into more and more dangerous routes. Truck smuggling is a way up," he wrote on Twitter.

White House press secretary Karine Jean-Pierre told reporters aboard Air Force One — where 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. "Our prayers are with those who tragically lost their lives, their loved ones as well as those still fighting for their lives. We're also grateful for the swift work of federal, state and local first responders."