MEXICO CITY, Dec. 26 (Xinhua) -- Mexican President Andres Manuel Lopez Obrador on Wednesday promised a transparent investigation into the helicopter crash that killed a couple who were the current and former governors of central Puebla state.

Lopez Obrador also urged patience over the investigative process and criticized the allegations from "certain right-wing groups."

"Be more responsible and less ridiculous," he said.

The helicopter carrying Governor Martha Erika Alonso, in office since Dec. 14, and her husband, Rafael Moreno Valle, who served in the post from 2011 to 2017, crashed on Christmas Eve shortly after taking off in the state capital Puebla.

"We are not going to hide absolutely anything so there is no suspicion. We have to find out what caused this accident, this regrettable tragedy," Lopez Obrador said at his daily press conference.

On Tuesday, the government announced it had invited international experts to take part in the investigation, including aviation officials from the United States. However, the U.S. federal government shutdown prevented their participation.

European and Canadian experts are expected to join in the investigation.

Mexican investigators have ruled out any possibility that explosives or fuel tampering played a part in the crash. Officials also confirmed the two pilots were experienced and the paperwork was in order.

The tragedy sparked speculations of wrongdoing by political opposition as Alonso's gubernatorial victory was questioned by the ruling Morena party, which alleged the elections had been marred by violence and vote tampering.

Alonso and her husband were members of the center-right National Action Party.