HOUSTON, Aug. 26  -- Nineteen students and a driver were sent to hospitals after the school bus they were riding slammed into a tree in the U.S. city of Houston Tuesday morning, local media reported.

The bus was on its way to the Paul Revere Middle School in West Houston at the time, and then veered off the road and hit a tree on the side, according to the Houston Chronicle, a local newspaper.

The 20 victims were taken to area hospitals with minor injuries, including scrapes, bumps and bruises, officials said. Eight other students were unhurt and were switched to another bus.

Local TV footage showed the yellow bus stopped against a tree, with its front bumper contorted and wing mirrors displaced. Emergency crews were seen carrying the injured on stretchers.

The exact cause of the crash is still unclear. The driver told the police that the bus had some "unexplained mechanical failure." But students said the driver, a woman in her 50s, seemed to have some kind of physical or medical issue moments before the crash. She was seen fall off her seat with a mobile phone in her hand.

Early Tuesday, three students in the Aldine Independent School District in north Houston were injured when a cement truck struck their school bus.