VENICE: Italian authorities on Wednesday were investigating what caused a horrific bus crash near Venice a day earlier, in which 21 people died including several children, and 15 were injured.

The electric bus, carrying foreign tourists returning from a day trip to Venice, crashed through the guardrail and off an overpass late on Tuesday in the Mestre district, slamming into the ground more than 10 metres (33 ft) below and catching fire.

The accident happened shortly before 8:00 p.m. (1800 GMT) on a straight and normally very busy road that connects Mestre to the historic centre of Venice.

Italian news agencies said that the dead included four or five Ukrainians, a German tourist and an Italian driver. ANSA, quoting emergency services, said a baby and a 12-year-old were killed.

Adnkronos, another news agency, said Germans, French, Croatians, Spaniards and Austrians were among the injured.

"We presume the driver may have fallen ill," Veneto regional president Luca Zaia told Rtl 102.5 radio, adding that witnesses' accounts and CCTV footage might give additional clues.

"At the moment we are not able to make a precise reconstruction of events," Venice Chief Prosecutor Bruno Cerchi told reporters overnight.

The wreckage of the bus, an electric vehicle that was about a year old, was cleared at dawn, and was still being monitored for fire risks from its batteries, firefighters said.

Italian Prime Minister Giorgia Meloni and politicians across Europe expressed their condolences.

"Our thoughts this evening are with the Italian people, the families and loved ones of the victims of the terrible tragedy in Venice", French President Emmanuel Macron wrote on X on Tuesday.

The head of the company that operated the bus, Massimo Fiorese, told Corriere della Sera newspaper he had seen video footage of the moments before the accident.

It showed the single-decker bus slowing down and appearing "almost stationary" when it crashes through the guardrail and falls down, he said.

"I think the driver had an illness, because otherwise I can't explain it," Fiorese said, adding the driver had started his shift less than two hours before the crash.

Footage of the accident site showed that the overpass was equipped with an old, relatively thin metal guardrail, rather than newer, sturdier concrete versions.