MEXICO CITY -- Federal authorities said Saturday they will take over the investigation into the massacre of 72 migrants at a ranch in northern Mexico because evidence suggests drug traffickers were responsible.

They also said an Ecuadorean migrant who was the lone survivor has refused Mexico's offer of a humanitarian visa and will return to his native country.

Government security spokesman Alejandro Poire said drug cartel involvement would make the killings a federal crime.

The government "will continue its frontal assault against these organizations so that terrible events like those that occurred this week will not be repeated," Poire said. One suspect, who claimed to be 16, was captured at the scene of the massacre and is in custody. Three other suspects and a marine were killed during a raid on the ranch.

Federal authorities said they will wait for survivor Luis Freddy Lala Pomavilla to recover from a gunshot wound in the neck and then help him leave Mexico.

Lala, who is under heavy guard, told investigators Monday that about 10 men who identified themselves as members of the Zetas drug gang traveling in five vehicles intercepted the migrants on a highway in Tamaulipas, a Gulf coast state bordering Texas.

They tied up the migrants, took them to the ranch and demanded they work for the gang, Lala told investigators. When most refused, they were blindfolded, ordered to lie down and shot.

Immigration Commissioner Cecilia Romero said Friday that Lala had been offered a humanitarian visa to stay in Mexico, but his mother said the 18-year-old begged her to arrange him to come to the United States, where she lives. The AP is not using the woman's name or her location to avoid putting her in potential danger.

The mother said she has been in contact with the Ecuadorean consulate, but officials there said they could only help him return to Ecuador.

Investigators have so far identified 31 of the dead: 14 Hondurans, 12 Salvadorans, four Guatemalans and a Brazilian.

They were the only ones carrying identifying documents, said Honduran Deputy Foreign Minister Alden Rivera. Investigators are collecting DNA from the rest, but Rivera said it might be impossible to identify many more.

In Honduras, worried relatives visited the Foreign Ministry seeking news on relatives believed to be in Mexico.

Maria Cruz was looking for word about her son, Denis Moreno, 34, who last contacted her from a city along the U.S.-Mexico border.

"I hope he is not on the list," she told a Honduran television station, sobbing. "I hope not."

Fabiana Carcamo told local media she had been notified that her 40-year-old brother, Miguel Angel Carcamo, died in the massacre. She said he left Honduras Aug. 3 and after some difficulties made it to Mexico. His plan was to reach the United States.

Carcamo left behind four children between the ages of 4 and 15 in his hometown of El Guante, about 35 miles (60 kilometers) north of the capital, Tegucigalpa.

"I talked with Miguel Angel on Aug. 3. He told me not to cry, that he would call me when he got there and that he would help me," said another sister, Ana Cristina.

Migrants frequently send money to support relatives back home.

Gangs have long kidnapped migrants and demanded payment to cross their territory. But the Mexican government says the cartels are increasingly trying to force vulnerable migrants into drug trafficking, a concern also expressed by U.S. politicians demanding more security at the border.

Mexican agents have rescued 2,750 migrants this year, some stranded in deserts and others held captive by criminal gangs, said Romero, the immigration commissioner.

In Tamaulipas alone, 812 migrants kidnapped by drug gangs have been rescued, she said. Many told authorities the cartels tried force them into drug trafficking.

The escalating danger has scared off many would-be migrants, resulting in a sharp drop in the number of people from Central America and elsewhere traveling through Mexico trying to reach U.S. soil, according to Romero.

The Zetas were founded by former Mexican army special forces soldiers and have become a lethal drug gang that has taken to extorting migrants.

The cartel controls much of Tamaulipas, a cattle-ranching state that is the last leg for migrants running the gauntlet up the Gulf coast to Texas.

Drug violence has spread throughout Mexico. On Saturday, police in the southern Pacific coast state of Guerrero found a man's dismembered body on the trunk of a car and his head on the roof.

The body was found near the state capital of Chilpancingo, along with a handwritten message. The area has witnessed a turf battle between several drug gangs that often leave notes at murder scenes that threaten rivals or officials.

On Friday, police found a total of 15 bodies throughout Guerrero state.