On 8 May, Mexico's top policeman, Edgar Millan, was shot dead outside his home in Mexico City.
It is the equivalent of killing the head of the Metropolitan Police in London, or the director of the FBI in the US.
Two other senior officers were then killed in the space of two days, the murders blamed on Mexico's powerful drug gangs.
The BBC News website looks at the violence and its impact in Mexico and also in the neighbouring US:
Acting police chief Edgar Millan was shot nine times at his home
The seriousness of the current violence in Mexico was thrown into stark relief this week when at least three high-ranking officers crossed into the US to seek political asylum because of the threats they had been facing from the drug cartels.
Seeking political asylum is, of course, usually associated with individuals fleeing persecution from governments and their forces of law and order, but in Mexico it seems it is the forces of law and order that are being persecuted.
In recent weeks, at least six senior police chiefs have been murdered.
The most prominent murder was that of Mr Millan, the acting head of Mexico's Federal Police Force (PFP).
At least one gunman was able to get into Mr Millan's home garage in Mexico City and wait for him. When he arrived, he was shot nine times.
The holes in the walls of his house at 132 Camelia Street show many bullets were fired.
It means not even the head of the 22,000-strong Federal Police Force is safe.
Mexican President Felipe Calderon, who attended Mr Millan's funeral, has spoken about "taking the streets back" from the cartels.
President Felipe Calderon has taken steps to try to curb the violence
An extra 3,000 or so army troops and federal police have been sent to the Pacific state of Sinaloa in another attempt by the government to stop the violence.
Police sources believe it was the Sinaloa cartel which organised Mr Millan's killing.
They may even have hired a serving federal police officer to carry out the attack. An officer suspected of being involved is among those who have been arrested.
But why shoot Edgar Millan in the first place? No one knows exactly.
He had been heading a force that had arrested dozens of members of the Sinaloa cartel. One of its leaders, Alfredo Beltran Leyva, was taken into custody in January.
That might have been motive enough for a cartel retaliation.
But there are other theories that are more calculating on the part of the cartels and so more chilling.
The aftermath of drugs violence in northern Mexico
According to George Friedman, an intelligence analyst for the Stratfor group: "The Sinaloa cartel struck in Mexico City not only to kill troublesome officials, but also to pose a problem for the Mexican government by increasing areas requiring forces, thereby requiring the government to consider splitting its forces."
Sophisticated stuff for what are effectively gangs of well-armed drug smugglers.
Mr Calderon has sent about 30,000 troops and federal forces to areas across Mexico to try to curb the violence. Splitting that deployment by creating more pockets of unrest would serve the cartels well.
The cartels' most dangerous elements include former army special forces troops who are collectively known as the Zetas.
They, more than most, would understand the importance of diluting an enemy's capabilities.
Shooting police officers and soldiers is not a new tactic.
In Ciudad Juarez, which borders the US city of El Paso, 12 police officers have been killed so far this year.
The police are targeted in part because they try to disrupt the cartels' activities. But, in some cases, it is because they themselves have been colluding with the gangs.
In March, soldiers in the northern state of Tamaulipas caught six local police chiefs at a petrol station sporting luxury watches and carrying envelopes stuffed with money.
Few doubt it was cartel cash. Their illicit drugs trade to the US is said to be worth $20bn (£10bn) - a lot of money to buy off bad cops.
On a much grander scale, the entire police force of Tijuana last year had their guns taken away because of suspicions some of those guns were being used for cartel business.
The guns were later returned.
Some optimists have suggested the cartels may finally be on the defensive - working together, partly out of desperation, to try to counter the government clampdown.
Smugglers have used tunnels to move drugs and guns across the border
There has been evidence of cartel co-operation in the past, but usually only to confront other cartels, not to gang up on the country's army and federal police.
And no-one is yet seriously suggesting that the Gulf cartel, the Sinaloa cartel, the Arellano Felix cartel and all the others have come to some sort of expedient understanding in a last-gasp attempt to save their operations.
Another theory is that the recent spate of shootings of officers may just be coincidence in a random selection of score-settling.
For example, one officer to die recently had his name on a hit list found on a police memorial several months ago.
Wherever the truth lies, it's rattling Washington.
"We are shocked by the escalating violence against Mexican law enforcement officials," said US State Department spokesman Sean McCormack.
He called the recent attacks "a brutal reaction to President Calderon's determination to fight organised crime".
These are not just soothing neighbourly words unmatched by actions. The US is now deeply involved in the fight against the cartels.
Operation Gunrunner, organised by the US Bureau for Alcohol, Tobacco, Firearms and Explosives (the ATF), is just one initiative currently in place between the two countries.
I don't think that generally the American public has any sense of the level of violence that occurs on the border
Jayson Ahern US Customs and Border Protection
Its aim is to block the smuggled arms trade between the US and Mexico.
Fully 95% of all guns being used by the cartels in Mexico originate in US gun shops. Many gun shop owners have been arrested, accused of being complicit in the trade.
Wittingly or unwittingly, Americans, it seems, are arming something approaching a war on their southern border.
"It's almost like a military fight," says Jayson Ahern, the deputy commissioner of the US Customs and Border Protection.
"I don't think that generally the American public has any sense of the level of violence that occurs on the border."
The violence has already spilled into the US. Seven front-line border agents were killed last year and two so far this year.
Some of that is the result of attacks by people-smugglers, but those same routes used to sneak in illegal migrants are also used by the cartels.
President George W Bush has requested $550m (£225m) to fight drug crime in Mexico. Congress is currently considering the proposal.
Mr Bush has asked the US Congress for cash to fight Mexican drug crime
It is not just to protect police officers and soldiers - civilians are being killed too.
Some 1,100 people - about nine a day - have been killed in Mexico so far this year in drug-related violence. Not all were police officers or cartel members.
Since 2006, just two and a half years, more than 6,000 people have died.
For comparison, in Iraq, the combined total of American and British military personnel deaths is around 4,200 since 2003.
"We have to come together to confront this evil, we Mexicans have to definitively and categorically say 'That's enough!'," Mr Calderon said.
But it doesn't appear to be enough for the cartels. Their ruthless pursuit of illicit drugs profits means anyone, in uniform or out, can end up as an addition to Mexico's escalating tally of violent statistics.
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