Police have captured four men accused of killing 12 federal agents and dumping their bloodied bodies along a highway in President Felipe Calderon's home state of Michoacan, federal authorities said Wednesday.

The head of the Federal Police's anti-drug unit, Ramon Pequeno, said the suspects belong to the La Familia drug cartel.

The federal officers were killed the weekend of July 12, when the gang retaliated for the arrest of an alleged gang leader by launching more than a dozen attacks on federal forces in one of the boldest offensives yet against the government.

At least 18 federal agents died in the attacks, in which gunmen threw grenades and fired on federal police stations.

Pequeno said one of the suspects, Jose Lopez, was caught Tuesday carrying one of the murdered agents' gun in the town of Arteaga, where the officers had been gathering intelligence.

The three other suspects were stopped by police while riding in a taxi Tuesday in Michoacan state, Pequeno said. He did not give further details.

Pequeno said Lopez told authorities the government's actions have left La Familia in a critical situation.

The government increased the presence of federal forces by 5,500 after the attacks. Federal police were told the top priority was arresting those who killed the agents.

Interior Secretary Fernando Gomez Mont said La Familia is acting out of desperation and he challenged the cartel to take on federal forces.

"Sirs, we are waiting for you," Gomez Mont said. "Mess with authorities, not with citizens. That's the invitation we're making you."

Michoacan has been at the center of Calderon's drug war. Calderon first sent troops to the state after taking office in 2006 to launch his organized crime crackdown.

In May, federal authorities arrested more than two dozen mayors and state officials in Michoacan for allegedly protecting La Familia in an unprecedented sweep against politicians.

Cartels have responded to the nationwide crackdown with a vengeance. More than 11,000 people have been killed by drug violence since 2006. The government says most of the victims are drug traffickers fighting rivals for lucrative drug routes to the United States.