A bus carrying Argentine frontier police plunged off a bridge in rural northern Argentina on Monday, killing 43 people on board, a spokeswoman for the provincial governor’s office told CNN.

The cause of the accident is under investigation but officials believe a tire may have blown out and caused the bus to veer off the bridge before dawn, the emergency chief for the province of Salta, Francisco Marinaro told AFP.

The bus, part of a three-bus convoy, came to rest in the Balboa River near the northern Argentina city of Rosario de la Frontera, roughly 1,160 kilometers (720 miles) northwest of Buenos Aires. Authorities said 51 people were on the bus.

Eight people were hospitalized, Lucía Mallozi, an official in the office of the governor of Salta province, told CNN.

Two were flown by helicopter to a hospital in the city of Salta, Mallozi said. The remaining six were being treated at a hospital in Rosario de la Frontera and their injuries were not considered critical, according to Mallozi.

President Mauricio Macri sent his condolences to the officers’ relatives and declared a day of national mourning.

“The Argentine people must stand with these families,” he said.

At the scene, Ernesto Flores, the provincial undersecretary for civil defense, had earlier reported 41 deaths and nine injuries.

Rescuers were trying to use a large crane to pull the bus from the river so they could retrieve bodies still trapped inside, according to Flores.

“There are still 30 bodies to retrieve. It´s complicated because it´s at the bottom of the river,” he said.

Authorities believe a tire blew on the bus, sending the vehicle careering off the bridge, Flores said.

The passengers were members of the Argentine National Gendarmerie, a national police force organized along military lines that provides border security and some domestic security functions.

“This has been the largest loss of life in the history of the force,” a border police spokesperson told CNN.