MEXICO CITY - Thousands of U.S. agents and local police officers arrested and interrogated suspected associates of Mexican drug cartels across the United States on Thursday in response to the killing of a U.S. anti-narcotics agent in Mexico last week.

Agents with the Drug Enforcement Administration launched the operation Wednesday after the Mexican military announced the arrest of eight suspects in the death of Jaime Zapata, a special agent for U.S. Immigration and Customs Enforcement who was killed in a roadside ambush. Zapata was buried in Texas on Tuesday.

DEA officials said the sweep netted more than 100 suspects - most of them low-level - in Atlanta, Oakland, St. Louis, Denver, Detroit, San Antonio, San Diego, Chicago and New Jersey, as well as in Colombia, Brazil and Central America. The DEA said officers in the United States have confiscated $8 million in cash, more than 100 weapons and 200 pounds of cocaine.

The DEA action, widely reported Thursday in Mexico, is intended to send a strong message to Mexican mafias that U.S. agents are off-limits, officials said.

"We're doing what we always do. But a message was sent. We want to make sure the traffickers understand that we're going to unite to go after them," said Derek Maltz, special agent in charge of the DEA's special operations division.

Late Wednesday, Mexican President Felipe Calderon announced the arrest of Julian Zapata Espinosa, who allegedly told investigators that the shooting was a result of "confusion" - that the gunmen either wanted to steal the armored sport-utility vehicle the agent and a colleague were traveling in or suspected that they were enemy cartel members.

Mexico's attorney general revealed that the leader of the Zeta drug cell accused in the killing was arrested by the military in December 2009 after being found in possession of high-powered weapons, camouflage uniforms and fake badges, but was released. In Mexico's dysfunctional legal system, many sensational arrests are followed by unpublicized releases.

Calderon and President Obama are scheduled to meet next week to discuss the U.S.-backed drug war in Mexico. Calderon this week defended the Mexican effort and labeled cooperation from the United States "notoriously insufficient."

"How can Americans cooperate?" Calderon asked. "By reducing drug use, which they haven't done. And the flow of weapons hasn't slowed; it has increased."