Bogota, Colombia ? Colombian flags hung outside two bars in a symbol of defiance Sunday after a suspected rebel tossed grenades that killed a woman and wounded at least 72 people, including an American Airlines pilot.

Police blamed the Saturday night attack on the nation’s largest rebel group, the Revolutionary Armed Forces of Colombia. They arrested a 25-year-old man they said threw the grenades at the Bogota Beer Company and Palos de Moguer, popular nightspots with locals and foreigners.

Besides the pilot, at least three other American citizens may be among the victims, said a U.S. official, speaking on condition of anonymity.

The United States has provided $2.5 billion to Colombia for its fight against rebels and drug traffickers, and the U.S. government is a supporter of hardline President Alvaro Uribe, but it was unknown if the Americans were targeted.

“It appears it was the FARC, like always,” National Police Chief Gen. Jorge Castro told reporters. However, the U.S. official said it was too early to tell whether the attack was politically motivated or a personal vendetta.

Colombia’s war pits the government and right-wing paramilitary groups against the Revolutionary Armed Forces of Colombia, or FARC, and a smaller rebel group in a conflict that kills about 3,500 people, mostly civilians, each year.

In a sign of defiance, a white banner reading “Never give in to the violent ones” hung outside Palos de Moguer.

“We cannot let the violence intimidate us,” said owner Guillermo Alvarez Forero. “We want the bar to be as it always has been, a calm place where people come to drink in peace.”

Expensive restaurants, a new shopping center and popular bars and dance clubs line the streets of the Zona Rosa, the downtown nightclub district. The two bars hit in the attack, both microbreweries, generally are packed on weekends.

Explosives experts inspect a bar damaged by an explosion in Bogota, Colombia. Police blamed members of Colombia's largest rebel group, the Revolutionary Armed Forces of Colombia, or FARC, for throwing explosives Saturday night at two crowded bars that are frequent gathering spots for foreigners in a popular nightclub district.

The revelry ended about 11 p.m., police said, when the suspected rebel threw the first grenade at the Bogota Beer Company, where young people filled an outside patio. The grenade caused a second explosion in a gas-powered heat lamp, said a witness, Jose Ramon Marceles.

The suspected rebel then allegedly ran to the nearby Palos de Moguer and tossed the second grenade, which witnesses said landed on a cloth awning and started a fire. People knocked over tables and bar stools as they fled the flames.

The alleged rebel tried to shove his way through the crowds, but a security guard grabbed him and turned him over to police, said Bogota Mayor Antanas Mockus.