By Duncan Kennedy
BBC News, Mexico City
Seven policemen died when they were fired upon during the raid
Seven policemen have been killed and four injured in Mexico's latest incident of drug-related violence.
The officers were killed during a raid on a home in Culiacan, in north-west Mexico, police said.
Arriving at the house to search for weapons and drugs, police were fired upon and a grenade was thrown at them.
Nearly 1,400 people have died this year across Mexico as the country's drug cartels fight among themselves and government forces.
That figure includes 450 police officers and other government officials.
Two people were arrested in the latest Culiacan raid, and seven AK-47 rifles were found.
The aftermath of drugs violence in northern Mexico
The city is in the state of Sinaloa and actions of the Sinaloa Drug Cartel make it one of the most violent places in Mexico.
The Mexican government sent 2,000 more troops and federal police there earlier this month to contain the trouble.
May has been a particularly bloody month for the authorities.
At least six senior officers have been murdered.
They include the director of police in the northern city of Ciudad Juarez and the acting director of Mexico's federal police.
Other senior officers have fled across the border into the US after receiving death threats.
Take back the streets
Around 30,000 troops and federal police have been sent to various parts of the country to reduce the violence.
But figures produced by the government last week show there's been nearly a 50% increase in cartel-related deaths compared with the same period last year.
Mexico's President Felipe Calderon has promised to take back the streets.
He claims the spike in violence is a sign his military clampdown is working.
Others, though, claim it shows the cartels are as audacious as ever, taking on the state itself in order to continue their $20bn-a-year narcotics trade to the US.