At least 19 people were killed when a bus flipped over in the state of Oaxaca in southern Mexico on Monday morning, just a few hours after an accident involving a bus and a tractor-trailer in northern Mexico resulted in the death of 14 passengers.

Mexican media reported that the victims in Oaxaca were supporters of the Morena party who were returning from President Claudia Sheinbaum’s Sunday rally in Mexico City.

Authorities were still investigating the cause of the accident, which occurred just outside the small town of Santo Domingo Narro, the Oaxaca government said in a statement.

Oaxaca Governor Salomon Jara expressed regret for the accident in a social media message posted on Monday:

“I extend my heartfelt condolences to the victims’ families, to whom we will provide the necessary support and assistance during this difficult time,” he said, adding that “Our government personnel continue to work and provide support to those injured.”

State Interior Minister Jesús Romero said the bus was carrying more than 40 people — including young children — and was en route from Mexico City to the Isthmus of Tehuantepec in the southern part of the state.

The newspaper El Universal reported that the bus had been rented by the United Front of Oaxaca Communities, an organization established by ruling Morena party Senator Antonino Morales, who shared his thoughts in a social media post:

“To the families and loved ones of those who have tragically passed away, we express our deepest condolences. Our deepest sorrow and solidarity are with you at this time. We assure you that you are not alone. We are working hand in hand with the relevant authorities and institutions to provide you with the care you require.”

Up north in the state of Durango, 14 people died after a tour bus crashed with a tractor-trailer and quickly caught fire near the town of Velardeña before dawn on Monday. Ten passengers managed to exit the bus but the intensity of the blaze prevented them from rescuing those still trapped inside.

The 10 survivors were transported to a hospital in the nearby town of Cuencamé, nine of them treated for minor injuries.

The bus — operated by Aser Tours of Durango city — was part of a round trip, two-day shopping excursion from Durango to McAllen, Texas. The bus had left McAllen for the return trip on Sunday night and was two hours northeast of Durango city when the accident occurred.

Aser Tours released a statement on its Instagram page saying it will offer support to victims. The bus company also claimed that the operator of the tractor-trailer was at fault in the crash.

Durango state authorities are still investigating the cause of the accident.

With reports from La Jornada, El Universal, Expansión and KRGV-TV