Two US Immigration and Customs Enforcement agents have been shot in Mexico, US officials have said.
The pair, described as "special agents" were attacked while driving between Mexico City and Monterrey. No details were provided on their condition or the precise location of the attack.
The investigative arm of Congress, the General Accounting Office, said the area lacked enough border agents to stop illicit activity such as gun running and illegal immigration.
'Line of duty'
The US Immigration and Customs Enforcement agency described the two victims as "special agents" based in the US capital, AFP reports. The agency said they were "shot in the line of duty".
The agency said it was working with the state department, US law enforcement and Mexican authorities to investigate the shooting.
Though there are high levels of violence among members of competing drug cartels within Mexico, US officials are rarely targeted.
However, in March 2010 a US consulate employee, her husband and a Mexican linked to the American consulate were killed by drug gang members in Ciudad Juarez.
The US government said it spent $3bn last year on controlling its southern border.
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.