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.”

Authorities in the nation’s fourth largest city are hunting a pair of killers – one of whom donned an alien mask – after a Texas convenience store clerk was shot and killed early Monday morning during a robbery.

The Houston Police Department said that two men entered a Valero gas station along Reed Road near Highway 288 at about 3 a.m. and fatally shot the clerk.

Surveillance images released by police showed the suspects entering the store, with weapons drawn.

One suspect can be seen wearing an alien mask, while the other suspect’s face was hidden by a more conventional black mask.

When officers arrived at the scene, they found the store clerk dead.

Police told FOX26 the suspects may have fled in a black 4-door sedan, possibly a Nissan.

Officials tweeted a hazy photo of the vehicle.