Speeding was fingered as a possible cause Monday of what is believed to be Japan's most expensive ever road accident when up to $4 million-worth of supercars ended up in a crumpled heap on a highway. Eight Ferraris and a Lamborghini -- plus a Toyota Prius -- were among the vehicles involved in the crash, which witnesses said happened when a speeding car slid across a wet road surface. Television footage showed mangled Ferraris -- many of them racing red -- and debris spread over some 400 metres (yards) of the east-bound side of the Chugoku Expressway, the main trunk road in southern Honshu. A pack of about 20 supercars was travelling in convoy on Sunday morning on a stretch of wet highway when the leading Ferrari slid into a guardrail, police said. Those behind slammed on their brakes, but for many of them it was apparently too late. "I've never seen such a thing," highway patrol lieutenant Eiichiro Kamitani told AFP by telephone. "Ferraris rarely travel in such large numbers." Ferrari Graveyard: Video of 14 supercar pile-up in Japan - YouTube Kamitani said 10 people -- five men and five women -- sustained slight injuries, in the accident. "It is highly possible that they were driving in couples." "Many of them were probably on their way to Hiroshima," some 130 kilometres (80 miles) to the east, for a gathering of supercars there, said Kamitani. "Speeding was possible but we have yet to determine the exact cause," he added. The Prius and a second Toyota also caught up in the 14-car smash were not thought to be part of the supercar pack. The three other vehicles involved in the accident were all Mercedes-Benz. An unidentified male eyewitness told the TBS network: "A group of cars was doing 140-160 kilometres (85-100 miles) per hour. One of them spun and they all ended up in this great mess." The speed limit on that section of the highway was 80 kilometres per hour. "The front car crashed into the left embankment and bounced off toward me," another man told public broadcaster NHK. One of the Ferraris was reported to be a F430 Scuderia, a model with a top speed of 320 kilometres per hour. Kamitani said the lead Ferrari was being driven by a 60-year-old self-employed man from Chikushino, near Fukuoka, on the southern island of Kyushu. Japanese media said the total cost of the pile-up could run to 300 million yen ($3.8 million), with new Ferraris retailing at more than 20 million yen each and Lamborghinis costing anything up to 30 million yen. Supercars are not necessarily owned by the super-rich in Japan. Many owners are young people who save up their earnings to satisfy their dream, according to media.