Mexican security forces have taken control of a city near the US border as part of an inquiry into alleged links between police and drug smugglers.
Troops surrounded the town hall of Nuevo Laredo, near the US border, and detained 41 policemen for questioning.
The crackdown comes after local police opened fire on federal agents sent in to investigate the murder of the newly-appointed police chief.
A surge of drug-related violence in the city has claimed 45 lives this year.
Nuevo Laredo is a major transit point for drugs entering the US and drug cartels have been fighting for control of the market.
Mexican President Vicente Fox launched "the mother of all battles" on the drugs trade, sending hundreds of armed police to the border cities in March to restore order.
But concerns about lawlessness were reignited last week with the killing of Nuevo Laredo's police chief a day after he took up the post.
Alejandro Dominguez's car was sprayed with bullets by attackers in an ambush.
On Saturday, federal agents sent in to investigate the killing were involved in a shoot-out with municipal police.
President Fox's government accused local police of being in the pay of drug traffickers.
"There are very clear signs of a relationship between elements of the Nuevo Laredo police and drug smuggling, hence the decisive action," said government spokesman Ruben Aguilar.
Troops in Humvee all-terrain vehicles patrolled the city's streets and special forces stood guard on street corners.
The detained officers were taken to Mexico City for questioning about the shoot-out, in which one federal agent was wounded.
Nuevo Laredo officials said local forces may have thought the federal agents were members of a drug gang posing as police officers.