Orlando, Fla. ? Packed into an airless truck trailer and abandoned to the South Texas heat, they pounded on the walls, begging for help, trying to escape.

For at least 18 of them, the aid, when it came, would be too late.

Police responding to a call from the truck stop on the outskirts of Victoria, Texas, arrived early Wednesday to find the bodies of 17 suspected undocumented migrants in and around the trailer, authorities said. One more died later Wednesday at a local hospital; another was in critical condition.

Apparently the victims of an aborted human-smuggling operation, they were thought to be part of a group of dozens — from Mexico, El Salvador, Guatemala and Honduras — who slipped into the United States from Mexico the night before. If so, the death toll would match the worst in U.S. history for migrants attempting to enter along the southwestern border.

Hours after the bodies were found, authorities arrested a suspect, thought to have been the driver of the 18-wheel tractor-trailer rig. He was said to be cooperating with investigators. Meanwhile, local, state and federal agents were searching for as many as 40 people who may have fled the trailer. By Wednesday afternoon, investigators had located about half; at least 14 were taken to local hospitals for medical attention.

Coming within months of the high-profile reorganization of federal border-protection agencies under the new Department of Homeland Security, the incident demonstrated again the challenges authorities face in trying to close the nation’s frontiers to illegal entry.

“We have been and continue to be at a heightened level of security in order to maintain a vigilant program of detection and deterrence,” said Jim Michie, a spokesman for the new Bureau of Customs and Border Protection. “We are charged with enforcing laws and we do the very best we can.”

The department consolidates the Border Patrol, the Customs Service, elements of the Immigration and Naturalization Service and other agencies in an effort to improve communications and pool resources. But critics say they have seen little change in enforcement.

“Our border security hasn’t increased as much as people think,” said Mark Krikorian, executive director of the Center for Immigration Studies. “I can’t think of any substantive change in border control. Mostly what’s happened is organizational change.”

Officials investigate the site where 18 people, believed to be illegal immigrants, were found dead at a truck stop near Victoria, Texas. The trailer in which some 60 immigrants were found locked Wednesday is at right; a mobile morgue is center.

The last great expansion of the Border Patrol came in the 1990s, when officials boosted the number of agents to more than 10,000, purchased more vehicles and aircraft, and acquired such technology as night-vision goggles and electronic sensors.

The number of undocumented migrants apprehended in the United States has dropped in recent years, from more than 1.6 million in 2000 to 930,000 last year.

It was not known where the migrants discovered Wednesday entered the United States.

The driver allegedly unhitched the trailer from his cab near Victoria, about 175 miles from the Mexican border, and drove off, leaving behind as many as 60 people packed into a space so stifling that they apparently tried to claw through the insulation on the door to get fresh air.

When the door was opened about 2 a.m. survivors spilled out into nearby fields and woods.

Authorities said one of three suspected smugglers was arrested Wednesday in the Houston area, about 115 miles northeast of Victoria. The suspected smuggler, of Schenectady, N.Y., was not immediately charged.