Gunmen tossed a fragmentation grenade and fired on a crowded taco stand Thursday in the central Mexican city of Uruapan, killing a police officer and a 15-year-old boy. Four others were injured.

A spokesman from the state prosecutor's office in Michoacan state, where Uruapan is located, said the policeman was shot while eating with a fellow officer.

Before fleeing, the assailants shot two tanks of cooking gas that exploded and burned to death the 15-year-old boy, a taco stand employee, said the spokesman, who was not authorized to give his name in line with the office's policy.

Officials initially reported three grenades were tossed and eight people were injured.

Investigators say the attack apparently was aimed at the two officers.

Uruapan has been plagued by drug violence. The city's mayor was among 10 Michoacan mayors detained by federal officials last month for alleged drug ties. In 2006, suspected La Familia cartel members dumped five human heads on a bar dance floor in the city.

Drug gangs have been launching increasingly bold attacks since Calderon launched a national crackdown on cartels in 2006 by sending troops to Michoacan, his home state.

Three alleged drug hit men remain jailed on terrorism charges for throwing grenades into crowds of Independence Day revelers in the state capital, Morelia, an attack that killed eight people and wounded 106.

More than 10,800 people have been killed by drug violence in Mexico since 2006.