Tien Giang police announced that the dump truck that killed three people on Sunday was hauling more than twice its legal limit.
Tests showed that the truck weighed 33 tons -- including a 20.3 load of sand -- when it hit four motorbikes, authorities said in an announcement issued on Monday.
The vehicle's maximum load allowed was 11 tons, police said.
Police have alleged that the truck was speeding before it struck four women who were stopped at a red light on National Highway 1A.
Their motorbikes were caught under the truck and dragged 40 meters down the road, they said.
Three of the drivers, 24, 25 and 39 years of age, were killed -- one managed to jump from her motorbike.
Cao Quoc Hien, the 32-year-old truck driver, fled the scene but later turned himself in to the police.
The case has been transferred to investigators at the Public Order Division of the Tien Giang Police Department.