Mexican President Andrés Manuel López Obrador on Monday called the case “painful” and “sad.”

Officials said late Sunday that Ambassador Óscar Ricardo Valero, 77, had resigned for health reasons. Mexico’s Foreign Relations Secretary Marcelo Ebrard wrote on Twitter that Valero “is undergoing neurological treatment.”

He also described Valero, a career diplomat with a long pedigree in Mexico’s left, as “a great person.”

Mexican news media reported that Valero allegedly also tried to steal a T-shirt from an airport shop after he was called back to Mexico in early December.

At the request of relatives, Ebrard’s office published a doctor’s letter saying Valero had been diagnosed with a brain tumor in 2012 that resulted in “behavioral changes.”

The news outlet, citing figures from the Secretariat of Security and Citizens Protection, noted that June saw the most murders of any month on record, with more than 3,000 people killed.

The violence has hit such a fever pitch that Mexico President Andres Manuel Lopez Obrador, who was sworn into office in December, has decided to send the National Guard to some areas, including Mexico City, a controversial move that critics say will not solve the high homicide rate and will instead lead to abuses.

And amongst the areas in the country that has the most homicides are Mexico City, Veracruz, Chihuahua and Jalisco, the news outlet reported.

The state with the highest murder rate is Guanajuato, where the Jalisco New Generation and Santa Rosa de Lima cartels are engaged in a power struggle, according to the BBC.

For years, Mexico put their trust on the military to confront organized crime, in some cases even disbanding local police forces because they had been infiltrated by cartels.

The increase in homicides stands in contrast to the vow made by Lopez Obrador’s campaign promise to reduce violence in the country.

In an interview with Univision’s Jorge Ramos, Lopez Obrador argued that security was improving in Mexico.

“Our data shows that we’ve brought this situation under control,” he said to Ramos.

But Ramos countered: “The data I have shows something else, you’re not controlling, to the contrary, many Mexicans continue to die.”

Security experts say fueling the rising violence is the fact that many cartels have splintered into factions that have engaged in increasingly bloody battles over control.