A US immigration and customs agent has been shot dead and a second wounded in an attack on their car in Mexico.

美国一名移民与海关负责人在墨西哥驾车途中遇袭被射杀，另一名官员受伤。

Officials say the pair, described as "special agents" shot "in the line of duty", were driving 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 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 (320 km) north of Mexico City

One of the agents was critically wounded and later died from his injuries while the second was shot in the arm and leg and remains¹ in stable condition, Ms Napolitano said.

The US Immigration and Customs Enforcement agency said it was working with Mexican authorities to investigate the shooting.

Border security

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 killing² 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.

However, in March 2010 a US consulate³（领事） employee, her husband and a Mexican linked to the American consulate were killed by drug gang members in Ciudad Juarez.

The attack came as a US report revealed that a fifth of the Mexico border, some 375 miles (600km), was not adequately（充分地） guarded.

The investigative arm of Congress, the Government Accountability Office (GAO), said the area lacked enough border agents to stop illicit⁴（违法的） 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 almost doubling 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.