BASTROP, Texas  — A cement truck on a Texas highway veered into a school bus carrying more than 40 prekindergarten students returning from a field trip to a zoo, killing a boy on the bus and a man in another vehicle, authorities said.

Officials said the bus rolled over on the rural outskirts of Austin, where a heavy presence of emergency vehicles shut down traffic for hours. The roof of the bus was crumpled, and much of another vehicle nearby was pulverized. Personal items were strewn across the road.

Four people in critical condition were airlifted from the crash site. Six others with potentially serious injuries were transported by ambulance, said Kevin Parker, division chief Austin-Travis County Emergency Medical Services.

An ambulance bus transported about 10 other patients to a children’s hospital with minor injuries, Parker said. Passengers on the bus included 44 students and 11 adults, according to the Hays Consolidated Independent School District.

The bus was struck at about 2 p.m. when a concrete truck traveling in the opposite direction veered into the bus’ lane, said Texas Department of Public Safety Sgt. Deon Cockrell.

Cockrell said that the man who was killed was in a vehicle that either ran into the back of the bus or maybe part of the concrete truck. He said there was one person in the concrete truck and one man in the other vehicle. Cockrell didn’t know how fast the vehicles were going.

The school district said the students attend Tom Green Elementary School in Buda, which is about 16 miles southwest of Austin.

The school district said parents of the students on the bus were notified of the accident and the district was working to reunite them.