Four people were arrested and $5 million in U.S. and Mexican currency was seized during the raid Monday in the industrial city of Monterrey, according to an army statement. Soldiers, acting on an anonymous tip, also seized drugs, money counting machines, cell phones and five vehicles.

Monterrey and the surrounding state of Nuevo Leon, which borders Texas, have been a focus of the federal government's crackdown on police corruption.

The cash and seized items were displayed at military barracks north of the city, with dozens of white envelopes containing some of the cash arranged in rows on a table.

Envelopes at the front of the rows had yellow post-it notes with the names of police precincts in Monterrey and other municipal forces in Nuevo Leon state. One was labeled "press."

Army officials refused to say more about what the money was intended for, declining to comment beyond the statement.

In June, nearly 80 police officers suspected of working with drug smugglers were arrested in 18 towns across Nuevo Leon after soldiers found lists of police names in the possession of traffickers.

Since then, Monterrey police have been banned from sitting in parked cars or using cell phones while on duty because of concerns they may be acting as lookouts for gangs. Police in Mexico's third-largest city have also been prohibited from setting up sobriety checkpoints because they allegedly used them to extort motorists.

The resort city of Cancun, meanwhile, fired 30 police officers in an effort to clean up the image of another force long plagued by corruption. Cancun Mayor Gregorio Sanchez said the city government "had lost confidence" in the work of the officers and some of them may have been working for criminal gangs.

The purge came a week after eight former top police and prosecution officials in Quintana Roo state, where Cancun is located, were ordered to stand trial on charges of aiding drug cartels.

Earlier this year, the Cancun police chief was arrested on suspicion of involvement in the assassination of an army brigadier general hired to root out corruption in the city.

President Felipe Calderon has acknowledged that corruption permeates Mexican police at all levels. He has relied on the army to fight ruthless drug cartels, deploying tens of thousands of soldiers across the country since taking office in late 2006. Gang violence has since surged, claiming more than 13,500 lives.

On Monday, gunmen stormed a motel room and killed two men and two women in the northern city of Ciudad Juarez, said Arturo Sandoval, a spokesman for the regional prosecutors' office.

Ciudad Juarez, across the border from El Paso, Texas, is Mexico's most dangerous city, with more than 1,600 people killed this year as gangs battle for lucrative trafficking routes into the United States.