LITCHFIELD, Conn. -- Three school buses carrying middle school children collided in a chain-reaction crash, sending at least 20 to area hospitals for observation.

A Connecticut State Police spokesman said the students Sage Park Middle School in Windsor were on a class trip to White Memorial Conservation Park in Litchfield when the accident happened about 1:15 p.m. Friday.

The buses, school officials said, were carrying 79 students and several staff members.

They said 19 students accompanied by staff members were taken to area hospitals. The parents of those students have been notified, they said.

Police said the first bus stopped for a vehicle turning into a Mobil gas station. They said bus two stopped behind bus one, which is when bus three rear-ended bus two. Bus two, they said, was pushed into bus one by the impact of the crash.

All students were expected to be released by late Friday afternoon.

One of the buses was towed due to damage, but others were released to the bus drivers, police said.

Police are investigating the incident.

The police spokesman said the cause of the incident was unknown.