In Guatemala on a Latin American tour, Cabral, 74, left his hotel at 5:40 a.m. in a white SUV for an eight-minute ride to the airport.

Gunmen attacked the SUV -- at least 20 bullet holes could be seen on the Range Rover. Nothing was reported stolen from the vehicle, government spokesman Ronaldo Robles said.

Police found a brown Hyundai Santa Fe nearby containing bullet-proof vests and AK-47 magazines.

A motive for the killing of one of Latin America's best-known folk singers remained unclear. Robles said an investigation was underway.

"Adios amigo!" said Argentine Foreign Minister Hector Timerman on Twitter.

Guatemalan Nobel Peace Prize winner Rigoberta Menchu traveled to the site of the attack, where she wept and said the singer had died "for his ideals," according to Notimex, the state-run news agency in Mexico.

"I can't think of a single reason why Cabral was killed here in Guatemala. He came just to sing," Notimex reported she said.

Cabral was the latest victim in a wave of violence that has rocked Guatemala ahead of elections.

In a report last month, the International Crisis Group warned that the violence and unregulated campaign finance were putting the country's political institutions at risk.

Stephen McFarland, the U.S. ambassador to Guatemala, echoed that warning Saturday.

"I think this of course hurts Guatemala's image before the rest of the region and I believe that brings serious questions to the table about what can be done to prevent this from continuing," he said.

Guatemala's human rights ombudsman, Sergio Morales, expressed his condolences to Argentina.

"I ask authorities of this country that this crime not be left unsolved, to investigate," he said.

Ironically, Cabral, who said he was inspired by Jesus Christ and Mohandas Gandhi, was recognized in 1996 by the Organization of the United Nations Educational, Scientific and Cultural Organization as a "World Peace Messenger."

Cabral gained fame as a protest singer. His song, "No Soy De Aqui, Ni Soy De Alla" ("I'm Not From Here Nor There") was recorded in nine languages by stars including Julio Iglesias and Neil Diamond.

*CNN's Claudia Dominguez and Moni Basu contributed to this report.