US Homeland Security Secretary Janet Napolitano has hailed the dedication of a US immigration agent killed in Mexico last week in a speech at his funeral.
"We know that no words or actions will fill the void left by Agent [Jaime] Zapata's actions or repay the sacrifice he made... ," Ms Napolitano said.
Hundreds attended the funeral in Brownsville, Texas. Mr Zapata was shot dead in San Luis Potosi state.
Another agent, Victor Avila, was wounded in the attack.
US Congressman Michael McCaul has said the pair were attacked after members of the Zeta drug cartel forced their vehicle off the road on a highway between Mexico City and the northern city of Monterrey.
'Eradicate the scourge that took his life'
"He [Mr Zapata] knew that if the good do not fight for what is good the enemy is aggressive and will take our children," Bishop Daniel Flores told mourners.
Ms Napolitano and US Attorney General Eric Holder vowed to continue helping Mexico in its war against drug cartels.
"We must and will eradicate the scourge that took his life," Mr Holder said.
The US Immigration and Customs Enforcement agency (ICE) said it was working with Mexican authorities to investigate the shooting.
Mr Zapata 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 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.
Mr Zapata was to be buried later on Tuesday in Brownsville - his hometown.