updated 9/26/2008 8:21:39 PM ET 2008-09-27T00:21:39

Mexican federal prosecutors have arrested three drug gang members accused of throwing grenades into crowds of Independence Day revelers, an unprecedented attack on civilians that killed eight people.

Prosecutors said the men, arrested Thursday, belonged to a group of infamous Gulf Cartel hit men known as the Zetas. Four others were under house arrest, but their involvement was unclear.

The announcement confirmed suspicions that drug gangs were behind the attacks. But it was still unclear why the Zetas would target seemingly innocent people for the first time.

Mexican President Felipe Calderon has sent more than 20,000 federal troops across the country to take back territory controlled by drug cartels. Gangs at war with each other and the government have responded with extraordinary violence — beheading rivals en masse, assassinating police commanders and ambushing patrols. Civilians are often caught in the crossfire, but rarely, if ever, deliberately attacked.

An anonymous call on Sept. 24 led investigators to a house in Apatzingan, a drug stronghold in Michoacan state. The three were arrested there, and officials said they confessed to throwing the grenades on the night of Sept. 15 in the western city of Morelia.

The suspects were paraded before reporters at Friday's news conference, but stated only their names. They did not give details of their alleged confessions or make any other statements.

Marisela Morales, deputy federal prosecutor for organized crime, said authorities still were investigating to corroborate the men's confessions.

The grenades exploded almost at the same time within blocks of each other. They came seconds after Michoacan state Gov. Leonel Godoy delivered the traditional "Viva Mexico" shout for independence from the balcony of the state capitol.

Thousands were packed into the historic city square to celebrate the 1810 start of Mexico's 10-year war of independence from Spain. The explosions injured more than 100, many of them children. The youngest person killed was a 13-year-old boy.

Michoacan, Calderon's home state, was the first place to receive federal troops after the president took office in December 2006.

Mexico's major drug gangs are fighting for control of lucrative smuggling routes in Michoacan, including the large Lazaro Cardenas port, stretches of Pacific coastline and remote pine-covered mountains. The fighting has made Michoacan one of the most violent states in Mexico, with frequent shootouts and gruesome decapitation-killings.

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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