A private Houston school bus struck a tree Tuesday en route to Revere Middle School, injuring 20 people.

There were 28 people aboard the bus when it veered off the road, mostly students headed to their second day of school. Eighteen students were transported to the hospital after the Houston school bus accident for minor injuries and checkups. The driver was also taken to the hospital for treatment.

According to a Houston Chronicle article, records show that the bus, operated by Nancy’s Bus Service, was not authorized by the state to be on the road. State records show that the company’s registration with the state was revoked in 2009 because of a lack of insurance. A Texas Department of Motor Vehicles spokesman said the agency would be investigating the company.

The school’s principal, Hafedh Azaiez, said he wasn’t sure why the students were using a private bus company. Houston Independent School District spokeswoman Shaleah Reed said the district doesn’t track private rides but that it’s not uncommon for parents to hire outside transportation. For example, students who don’t live in an area that qualifies for HISD busing, or who don’t like the district’s pickup location.

Jodi Silva, a spokeswoman with the Houston Police Department, said the driver blamed the accident on a mechanical failure. However, in a KHOU news report, students who were aboard the bus said the driver appeared to have a physical or medical issue immediately before the accident.