MEXICO CITY – Federal investigators suspect the powerful La Familia drug gang was behind a grenade attack that killed seven people in an Independence Day crowd and increased violence-scarred Mexicans' sense that no place is safe.

A spokeswoman for the Attorney General's Office said Thursday that investigators have uncovered evidence pointing to the gang, based in Morelia where the attack occurred. She would not elaborate.

It remains unclear why drug traffickers would turn from killing their enemies to carrying out attacks against the general public, something unheard of in Mexico.

The spokeswoman, who was not authorized to give her name, said authorities also are investigating a text message claiming La Familia did not carry out the attack Monday night, in which two fragmentation grenades killed seven people and injured 108.

The message, sent to the cell phones of reporters and Morelia residents, blames Gulf Cartel hitmen known as the Zetas, raising doubts about a supposed alliance between La Familia and the Gulf Cartel.

The spokeswoman said investigators were trying to determine whether the messages really came from La Familia, a ruthless gang that emerged a few years ago as cartels battled for control of Morelia.

Meanwhile, federal investigators planned to question two men detained by the army early Thursday in the northern state of Zacatecas to determine whether they were involved in the attack, Interior Secretary Juan Camilo Mourino told reporters. He did not elaborate.

Since Wednesday, police have been searching for a tall, heavyset man, using a composite sketch provided by witnesses who saw him lob a grenade into the crowd, then beg for forgiveness before slipping away.

But until Thursday, federal prosecutors had insisted they did not have enough evidence to tie the attack to organized crime, even though local officials and U.S. Ambassador Tony Garza publicly blamed Mexico's warring cartels.

The Gulf Cartel is increasingly under pressure from the Mexican, U.S. and Italian governments. U.S. authorities on Wednesday announced the arrests of more than 500 alleged members of the gang in the United States and Italy, and indictments against three alleged leaders of the cartel in Mexico.

U.S. Drug Enforcement Administration spokesman Garrison Courtney said he did not believe the Morelia explosions were in retaliation, because the indictments were handed down after they occurred. But he said the violent gang is under extreme pressure.

Drug gangs are believed to be battling for control of Michoacan state, of which Morelia is the capital. Lucrative drug routes include the state's port of Lazaro Cardenas, its remote Pacific coastline and its relatively unpopulated pine-covered mountains.

President Felipe Calderón has sent more than 25,000 troops to drug hotspots around the country, beginning in late 2006 with Michoacan. On Wednesday, he traveled to Morelia – his hometown – and pledged “the full force of the state” in finding those responsible.