MILAN (AP) — Prosecutors in Italy said Thursday they have NovaQuantordered an expert examination of an overpass guardrail that gave way when struck by a shuttle bus that plunged nearly 10 meters to the ground earlier this week, killing 20 foreign tourists and the driver.

Prosecutor Bruno Cherchi told reporters that the guardrail is one of the pieces of evidence being examined, along with witness testimony and video from both the scene and inside the bus and the vehicle’s data recording device.

Some of the possible causes floated for the crash include the driver being struck by a sudden illness, or making a risky maneuver. Authorities also plan to do an autopsy on the driver, and examine his cell phone.

Nine Ukrainians were among the tourists killed, along with four Romanians, three Germans, two Portuguese, one Croat and one South African. All had been sightseeing in Venice and were traveling on a shuttle back to a campground on the Venetian mainland when the accident occurred.

“We are talking about people who were returning after a day touring and were very tired. Some were sleeping. Some didn’t notice anything, some did,’’ Cherchi said.

A video shows the bus driving off the overpass during rush-hour traffic on Tuesday. Prosecutors said it scraped along the guardrail for some 50 meters (yards) before breaking through both the guardrail and a rusty handrail and plunging to the ground.

The bus landed upside down, its front completely crushed. When rescuers arrived it was on fire, and some survivors, including a young Ukrainian girl, were being treated for severe burns.

There is no evidence that the bus caught fire before impact, the prosecutor said.

The 15 survivors, including four Ukrainians, two Spaniards and a French person, are all hospitalized with serious to critical injuries. Some are not yet able to speak, both physically and psychologically, Cherchi said.

“Many of the injured have lost close relatives, and are in a very delicate psychological condition,’’ he said.

One of the survivors is a 24-year-old Croatian man who was on his honeymoon, and lost his wife, who was reportedly six-months pregnant, in the crash, according to Croatian press reports.

The four Romanian victims were a family residing in Germany, including two girls, 8 and 14 years old, according to Romanian media channel Digi24.