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.”