A southern Mexican town's 15-member police force has quit for fear of being assassinated in retaliation for a shootout with gunmen, a security official said Thursday.

Zirandaro was the second town in less than two weeks to be left without its police force as Mexico's drug cartels wage increasingly bold attacks against security forces. On Monday, the military took over a town near Texas after all 20 of its police officers were either killed, run out of town or quit.

Eight members of Zirandaro's police never returned to work after a May 13 shootout with gunmen that left a 32-year-old man dead, said Juan Heriberto Salinas Altes, the public safety secretary of the southern state of Guerrero.

The other seven officers — including the police chief — quit days later.

"The Zirandaro police quit the service because they feared the criminals would return to seek revenge," Salinas Altas told a news conference.

The identities of the gunmen were not known, but Salinas Altas said cells of both the Sinaloa and Gulf cartels were operating in the area.

About 20 Guerrero state police officers have taken over security responsibilities in Zirandaro, a town of about 24,000 people.

President Felipe Calderon has said the attacks against Mexican police show that cartels feel threatened by his crackdown against drug trafficking. Since taking office in 2006, he has sent more than 25,000 troops to drug hotspots.

But the disintegration of two municipal forces shows how vulnerable police feel in a country where, despite efforts to fight corruption, they can't be sure their colleagues are not on the cartels' payrolls.

Earlier this month, Mexico's acting federal police chief was killed in his home by an assassin who had keys to his house. A fellow federal police officer and four other people with alleged ties to the Sinaloa cartel were arrested in the killing.

President Bush's administration has pushed Congress to approve an initial $550 million to help fight drug crime in Mexico and Central America.

The U.S. Senate approved only $450 million for the plan on Monday, while the House has approved $461.5 million.

The U.S. ambassador to Mexico, Antonio Garza, nonetheless said approval of the funding "signaled congressional support for this important measure to enhance ongoing U.S. programs for cooperating and coordinating with the Mexican government."

The two chambers must agree on a final version of the bill before sending it to Bush for final approval.