A US immigration and customs agent has been shot dead and a second wounded in an attack on their car in Mexico.
Officials say the pair were shot "in the line of duty" on a road in San Luis Potosi, between Mexico City and the northern city of Monterrey.
Some reports suggest they were attacked after stopping at what appeared to be a military checkpoint.
US Homeland Security Secretary Janet Napolitano said she was "deeply saddened" by the attack.
"Any act of violence against our personnel... is an attack against all those who serve our nation and put their lives at risk for our safety," she said.
She added that the US was committed to supporting Mexico's efforts to combat violence within its borders.
Tuesday's attack occurred near the city of San Luis Potosi, some 200 miles (320km) north of Mexico City.
One of the agents, Jaime Zapata, was critically wounded and later died from his injuries while the second, as yet unidentified, was shot in the arm and leg and remains in a stable condition.
The US Immigration and Customs Enforcement agency (ICE) said it was working with Mexican authorities to investigate the shooting.
In a statement, the Mexican foreign relations secretariat condemned the attack and said federal and state authorities would work together to investigate.
Meanwhile, the Dallas Morning News, citing an unnamed US law enforcement official, said the agents were probably unarmed and were shot as they approached an unofficial checkpoint apparently run by members of a paramilitary drug gang.
Mr Zapata, a native of Texas, joined ICE in 2006 after working as a border patrol agent.
He was assigned to the agency's human smuggling and trafficking unit, and was recently on attachment to the ICE office within the US embassy in Mexico City, the agency said in a statement.
The BBC's Julian Miglierini in Mexico City says parts of the road where the incident occurred - Highway 57 - are regarded as dangerous, and it is not yet clear whether the killing was drug related.
In the past four years, some 34,600 people have died in drug-related violence, according to official government figures.
Though there are high levels of violence among members of competing drug cartels in the country, US officials are rarely targeted.
In March 2010 a US consulate employee, her husband and a Mexican national were killed by suspected drug gang members in Ciudad Juarez but although there have been arrests in that case, the motive for the shooting is still unclear.
Tuesday's murder 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 while on assignment in Mexico. The murder of agent Enrique "Kiki" Camarena strained US-Mexico ties at the time.
The latest attack came as a US report revealed that a fifth of the Mexico border, some 375 miles (600km), was not adequately guarded.
The investigative arm of Congress, the Government Accountability Office (GAO), said the area lacked enough border agents to stop illicit activity such as gun-running and illegal immigration.
The GAO report said the US had spent $3bn last year on controlling its southern border.
It said it had been able to increase the area under control by building more fences and increasing the number of agents from 10,000 to 17,500.
Security at the US-Mexico border has long been a hot political issue, with many in the southern United States calling for more to be done to stop drug smugglers and human traffickers from entering the US.