MORELIA, Mexico — Police searched Wednesday for a tall, heavyset man, using a composite sketch provided by witnesses who saw him lob a grenade into an Independence Day crowd, then beg for forgiveness before slipping away.
    Local officials and the U.S. ambassador insisted Mexico’s warring drug cartels were behind in the attack that killed seven people Monday night, but federal prosecutors who took over the case said they did not have enough evidence yet to link the attack to organized crime.
    ‘‘I believe the narco-terrorists have gravely underestimated the courage, valor, and strength of the Mexican people,’’ U.S. ambassador Tony Garza said in a statement, adding: ‘‘They have crossed a line from recklessly endangering civilians in their attacks on law-enforcement officials and rival gangs, to deliberately targeting innocent men, women, and children.’’
    In Atlanta, meanwhile, U.S. authorities announced the arrests of more than 500 alleged members of Mexico’s violent Gulf Cartel, and indictments against three alleged leaders of the cartel in Mexico. But while the Gulf Cartel has a strong hold on parts of Michoacan, U.S. officials said they did not believe Monday’s attack was related.
    ‘‘They wouldn’t have known about this, since the indictments weren’t handed down until this morning,’’ Drug Enforcement Administration spokesman Garrison Courtney said.
    However, he cautioned that people should expect the worst from the exceptionally violent gang.
    ‘‘We’re mindful they might try to retaliate,’’ Courtney said. ‘‘We’re preparing for that.’’
    Two fragmentation grenades blocks apart were used in the attack on a family friendly ceremony marking the start of Mexico’s 1810 war of independence. Seven people were killed and 108 were injured, and Mexicans’ already shaky sense of safety was further rattled.
    ‘‘Mexicans will not be the same after these cowardly acts,’’ Michoacan Gov. Leonel Godoy said Wednesday. ‘‘But the people of Michoacan and those of Mexico are more than a cowardly act. We know how to rise above this.’’
    Michoacan is at the center of Mexico’s drug wars. Two of the main drug gangs are believed to be battling for control of lucrative drug routes that include Michoacan’s Lazaro Cardenas port, its remote Pacific coastline and its relatively unpopulated pine-covered mountains.
    Calderon has sent more than 25,000 troops to drug hotspots around the country, beginning in late 2006 with Michoacan. After Monday’s attack, he pledged an immediate military response, and he headed Wednesday to Morelia — his hometown — to announce the details.
    The attack on the general public appeared to represent a new tactic in the intensifying war between drug cartels and federal authorities. Previously, most violence — including assassinations, massacres and beheadings — targeted rivals, police or soldiers.
    Witnesses said a man in his late 20s dressed in black threw a grenade, then begged for forgiveness as another grenade exploded nearby. They provided enough details for a composite sketch, which was distributed to police agencies in surrounding states, according to Michoacan Attorney General Miguel Garcia.
    He announced a 24-hour hotline to handle the flood of anonymous tips, many of which he called unsubstantiated, and urged the public to come forward with any photos or video taken during the attacks.
    The federal Attorney General’s office, which is leading the investigation, said authorities are looking into illegal arms charges, but do not yet have enough evidence to say for sure that drug cartels were responsible. No arrests have been made and officials declined to say if anyone had claimed responsibility.
    Mexicans said the attack made them feel more vulnerable than ever.
    ‘‘Before we were safe, but now we walk around afraid,’’ said Sandra Munoz, 22, who stopped in a Morelia church on her way to work to light a candle for the dead. ‘‘Everyone is unsafe now.’’
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    Associated Press writers Julie Watson and E. Eduardo Castillo contributed to this report from Mexico City.