Three policemen have been killed in a shootout at Mexico City's main airport, officials say.
The security ministry said the federal officers had been sent to detain suspects in a drug trafficking case.
Passengers dived for cover when the suspects opened fire, killing two officers immediately and injuring a third who later died in hospital.
The area was sealed off but flights were reported to be operating normally in and out of the airport.
Witnesses said the shooting happened in the food hall of Terminal 2.
"When the alleged perpetrators were surrounded by the police, shots were fired against the federal agents," a security ministry statement said.
Police would not say if any arrests had been made.
Eyewitness Israel Lopez told the Associated Press news agency that those who fired at the federal agents were wearing police uniforms.
"We were in the food court, and some policemen came in and started shooting at another policeman who was on the floor," he said.
The federal public safety department said it was investigating whether the attackers were active-duty police, former officers or impostors, AP reports.
The BBC's Will Grant in Mexico says shootings in public spaces in broad daylight remain rare in the capital city, which has been largely insulated from the violence seen elsewhere in the country.
Though traffickers use the capital's main airport to move drugs, money and illegal migrants and have seized 440lb (200kg) of cocaine there so far this year, violence related to drug trafficking seldom occurs in passenger areas.
Reuters reports that gang violence has been growing in Mexico City and its suburbs, with around 300 gang-related murders last year.