WASHINGTON -- Federal, state and local authorities conducted a massive sweep Thursday of suspected Mexican drug cartel members in the United States in a widespread domestic response to the killing of a U.S. agent in Mexico last week.

"We are taking a stand and we are sending a message back to the cartels that we will not tolerate the murder of a U.S. agent, or any U.S. official," said Carl Pike, assistant special agent in charge of the Drug Enforcement Administration's special operations division.

Pike said the nationwide roundup, which began Wednesday and was expected to continue into Friday, is targeting suspected criminals with ties to any Mexican drug cartel to try to disrupt drug trafficking operations in the United States.

By Thursday morning, agents in areas including Atlanta, St. Louis, Denver, Detroit, San Antonio, San Diego, Chicago and New Jersey had seized more than $4.5 million in cash and nearly 20 guns, arrested more than 100 people and confiscated about 23 pounds of methamphetamine, 107 kilos of cocaine, 5 pounds of heroin and 300 pounds of marijuana at more than 150 different locations.

Immigration and Customs Enforcement agent Jaime Zapata was killed, and fellow ICE agent Victor Avila was wounded in Mexico on Feb. 15 when the Chevy Suburban they were in was run off the road by at least two vehicles loaded with armed men. Authorities have said the agents, who were driving in a fortified sport utility vehicle with diplomatic license plates, identified themselves as U.S. diplomats in the moments before the shooting.

Mexican authorities have arrested one person in connection with the brazen attack, which is believed to be the work of members of Mexico's Zetas gang. Former Mexican special forces soldiers are among its members.

"We are basically going out to disrupt narcotics distribution here in the United States no matter what cartel their allegiance is to," Pike said. "It would be futile to send a message back to one cartel when they all are just as guilty."

Pike said that while the sweeps are a direct response to Zapata's killing, the majority of suspects were already targets of other investigations.

"People actually sacrificed a great deal of work" for these sweeps, Pike said. "For the lost agent's memory it's important, but we're also in a bully situation. If we don't push back, some other 18-year-old cartel member is going to think, 'They didn't do anything, so all U.S. citizens are fair game."'

Mexican law enforcement and politicians have become routine targets of Mexico's warring drug cartels. But for the most part, U.S. authorities had largely been avoided.

Last year, an employee at the U.S. consulate and her husband, a Texas jail guard, were killed on their way back to El Paso, Texas, from a birthday party in neighboring Ciudad Juarez. But Zapata's killing marks the highest profile attack on U.S. authorities in Mexico since the 1985 kidnapping and killing of DEA agent Enrique "Kiki" Camarena.

This week, Mexican President Felipe Calderon complained that U.S. help in the fight against warring cartels has been "insufficient." He made the comment to a Mexican newspaper in response to recently disclosed secret diplomatic cables in which U.S. officials criticized his anti-drug strategy. Calderon told the newspaper that the dispatches show U.S. diplomats' ignorance about Mexican security efforts.

More than 35,000 people have been killed since Calderon launched a crackdown against drug gangs in December 2006.

Mexican authorities are leading the investigation of Zapata's death and the Justice Department has announced a joint task force, led by the FBI, with the Homeland Security Department.

The Drug Enforcement Administration coordinated the sweep along with Immigration and Customs Enforcement.