Experts probing the cause of Sunday’s derailment of a high-speed train in Spain, which killed at least 39 people, found a broken joint on the rails, according to a source briefed on initial investigations into the disaster.

The derailed carriages smashed into an oncoming train, pushing it off the tracks and down an embankment in one of the worst train disasters in Europe in modern times.

The accident happened near Adamuz, in the southern province of Cordoba, about 360 kilometres south of the capital, Madrid.

Technicians on site analyzing the rails identified some wear on the joint between sections of the rail, known as a fishplate, which they said showed the fault had been there for some time, the source said.

They found that the faulty joint created a gap between the rail sections that widened as trains continued to travel on the track.