Rescue workers are seen on the site of collision between train and school bus in Millas, France December 14, 2017 in this handout picture. Must Credit France Bleu Roussillon/Handout via REUTERS

PARIS (Reuters) - Investigators on Friday reconstructed the seconds before a train plowed into a school bus at a level crossing in southern France and killed at least five children, an incident the prime minister described as “horrific”.

The collision on Thursday afternoon ripped the bus apart and authorities worked into the night to identify the first victims.

The official death toll came after a source close to the investigation earlier said six youngsters had lost their lives in an incident that stunned the small community of Millas, near Perpignan.

Hospitals appealed for blood to help them treat seven others who remain in a critical condition. Government spokesman Benjamin Griveaux said the cause of the crash was still unclear. It occurred at a crossing which was not considered dangerous.

Investigators are looking into whether the automatic barriers and lights on the level crossing malfunctioned and have taken blood samples from the driver for toxicology studies, French media reported.

The driver, a 49-year-old woman who was injured in the collision, was described “lucid” by the manager of the bus company in an interview with BFM TV. The bus was well maintained, he added.

The train was carrying 25 passengers and traveling at 80 kmh, the regulatory speed for the section of track where the collision occurred, the national SNCF railway said.