Editor's note: Read more about the accident at Louisian's WAFB.com.
(CNN) -- A tractor-trailer slammed into a school bus driven by the mayor of a Louisiana town Friday morning, critically injuring two children.
Twenty-one children -- ranging in age from 5 to 17 -- were on the St. Landry Parish school bus when it was hit while pulling back onto a highway in central Louisiana, Louisiana State Police said in a news release.
Earlier Friday state police indicated that the crash left three children -- ages 5 and 15, plus one more whose age was unknown -- in critical condition. As of 5 p.m., Port Barre High School Principal William Duplechain said, 19 students had been treated and released from hospitals, and two remained in critical condition.
They included a kindergarten student, who was unresponsive, according to the principal. The other was a middle-schooler who was placed in a medically induced coma so surgeons could operate on his broken leg.
Gil Savoy, a substitute bus driver, was driving the students when the accident occurred, Port Barre Town Clerk Toria Comeaux said. Savoy is the mayor of Port Barre, a community of about 2,400 people some 30 miles north of Lafayette.
He had pulled over on the side of U.S. 190 and, after all the students had boarded, began to slowly move forward "with all of his warning devices still activated," according to Louisiana State Police.
At that point, an 18-wheel truck driven by David Cox, 52, of Dallas, Texas, rammed into the bus from behind, police said. Comeaux said that the truck, which she was driving behind, had debris coming off it at the time of the crash.
Both Savoy and Cox suffered minor injuries, with "routine toxicology samples" taken from both, the state police said. No one has been charged, and the investigation is ongoing.
Video from CNN affiliates KLFY and KATC showed severe damage to the back of the bus and to the tractor-trailer's front.
Anthony Standberry, a St. Landry Parish school board member, told KLFY that the speed limit on that stretch of U.S. 190 is 65 mph. He said he favors working with parents and school officials to remove all bus stops along major highways.
CNN's Max Newfield and Vivian Kuo contributed to this report.