Authorities in Texas are investigating the deaths of 46 people after they were discovered inside a tractor-trailer in San Antonio on Monday, one of the highest death tolls involving migrants crossing into the United States.

Federal investigators are working with local and state police to identify the victims. Three people have been detained so far and local police planned to search the site again for more bodies on Tuesday.

Here’s what we know so far.

The investigation will continue Tuesday.

The mayor of San Antonio, Ron Nirenberg, called the deaths a “horrific human tragedy” at a news conference on Monday evening. The police chief, William McManus, said three people had been detained.

At least 16 people are in the hospital, including some children, who appeared to be suffering from “heat stroke, heat exhaustion,” said Charles Hood, the fire chief of San Antonio.

All the victims are believed to have been brought into the United States illegally through the border with Mexico. The closest border crossing is about 140 miles away.

Where are the migrants from?

Authorities have yet to identify any of the victims. But officials from the governments of Honduras, Guatemala and Mexico said on Monday night that they were investigating.

Violence in Central America, especially tied to gangs, has forced people to make the trek to the United States through Mexico. Climate change has also played a role.

A nearby worker discovered the truck.

A worker from a nearby business heard a cry for help and went out to investigate, Chief McManus said. The worker found the doors of the trailer partially open and bodies inside. The truck was found on a road sandwiched between train tracks and auto salvage yards. Several farms are nearby.

A local couple who rushed to the scene to pray for the victims after they were discovered said the area was known as a “drop-off spot” for migrants.

How common is this?

The journey north for migrants crossing into the U.S. from Mexico is typically dangerous, and sometimes fatal. Smugglers often transport large numbers in trailers, vans or S.U.V.s through remote areas in sweltering weather. And in recent years, dozens of migrants have died in crashes while traveling north.

San Antonio is a major transit point for migrants making their way from Texas to places across the United States. Tens of thousands of migrants have passed through the city in recent months, according to immigrant advocates.

What’s the situation on the border?

The number of migrants stopped at the southwestern border rose in May to the highest in decades, according to Customs and Border Protection data. In recent days, law enforcement officials along the border and in nearby counties have expressed concern at the number of arriving migrants in Texas, which has long been one of the most heavily trafficked borders for migrants.