Rome (CNN) -- Thousands of mourners gathered in southern Italy on Tuesday for the funeral of 38 people killed when their bus plunged off a bridge as it returned from a visit to a Catholic shrine.
Italian Prime Minister Enrico Letta was among those who attended the service in the village of Pozzuoli, near Naples.
He said on Twitter that Tuesday would be a national day of mourning for the victims.
An estimated 5,000 people turned out for the funeral, held in a sports arena, a spokeswoman for the Naples police said.
The bus was traveling east of Naples in Avellino province when it crashed late Sunday. The passengers had participated in a weekend pilgrimage to the Padre Pio shrine in Pietrelcina.
Rosario Cantelmo, the Avellino prosecutor who is coordinating the investigation, told CNN that the bus driver -- who was among those killed -- is the focus of inquiries.
An autopsy on the driver's body should be completed in the coming days, Cantelmo said.
Andrea Regione, a photographer for the Corriere dell'Irpinia newspaper, told CNN that the bus went off a bridge and fell roughly 30 meters (almost 100 feet) into a heavily wooded area below.
Photos from the scene showed a broken guardrail and the bus lying on its side.
Avellino police official Pasquale Picone said the bus struck 11 cars on the road before falling off the bridge, leading police to suspect the bus had brake problems. But the exact cause of the accident remains unknown.
The funeral in Pozzuoli comes only a day after a somber memorial Mass in Spain for 79 victims of last week's train crash outside Santiago de Compostela.
Journalist Barbie Latza Nadeau contributed to this report.