Mexican troops have arrested an alleged drug gang hitman suspected of killing a US immigration agent last week.
Julian Zapata Espinoza told soldiers he and other gunmen opened fire on the agent's car, thinking it belonged to a rival gang, officials said.
Agent Jaime Zapata was killed and agent Victor Avila injured in an ambush on a road near the city of San Luis Potosi.
Drug violence is set to be discussed when Mexican President Felipe Calderon visits President Obama next week.
Mexican officials presented six men to the media during a news conference on Wednesday, accusing them of involvement in the attack on the US agents.
These included the alleged leader of the gang, Julian Zapata Espinoza, known as El Piolin or Tweety Bird.
Three women, among them Mr Zaptata's wife, and a child, were also detained.
'Confusion'
Officials said the alleged gunmen ambushed the US agents as they were driving on a road near San Luis Potosi, some 200 miles (320km) north of Mexico City.
There was confusion, "given the characteristics of the vehicle in which they [the agents] were travelling and they [the suspects] thought they were working for a rival gang," army spokesman Col Ricardo Trevilla said.
He said Mr Zapata had admitted being the head of a cell working for the Zetas drug gang.
It has also emerged that Mr Zapata had been arrested in December 2009 but had jumped bail.
The Zetas and a rival gang, the Gulf Cartel, are vying for control of the San Luis Potosi area, which is also on the route used by illegal migrants seeking to reach the US.
Drug gangs are also increasingly involved in abducting migrants, often to extort money from their families.
Strains
The White House said President Obama had spoken with President Calderon to thank him for the efforts to bring the attackers to justice.
The two men are set to discuss ways of tackling drug-related violence, amid tensions over US-Mexico co-operation on the issue.
President Calderon said ties were strained after WikiLeaks released diplomatic cables, in which US officials criticised Mexico's anti-narcotics efforts.
The murder of Jaime Zapata was the highest-profile killing of a US official in Mexico since 1985, when an undercover Drug Enforcement Administration (DEA) agent was kidnapped, tortured and killed.
Mr Zapata, a 32-year-old Texan, was assigned to the Immigration and Customs Enforcement Agency (ICE) human smuggling and trafficking unit.
He had been on attachment to the ICE office within the US embassy in Mexico City.
More than 34,000 people have been killed in drug-related violence in Mexico since December 2006, when President Calderon's government deployed soldiers and federal police in a widespread crackdown on the illegal cartels.
source : bbc.co.uk
