Three young children were struck and killed by a pickup truck in northern Indiana as they were crossing a road to board a school bus, the state police said. The police identified the victims as a 9-year-old girl and her twin 6-year-old brothers.

"I just can't imagine that pain," Sgt. Tony Slocum told CBS affiliate WSBT-TV. "I haven't seen troopers cry in a long time, but I saw that today."

Slocum said the girl and her brothers died at the scene Tuesday morning after they were struck by the pickup near Rochester, about 100 miles north of Indianapolis. An 11-year-old boy who isn't a relative was also struck and suffered multiple broken bones.

That boy was conscious and speaking to emergency workers before he was airlifted to a Fort Wayne hospital, Slocum said. The children were hit as they were crossing a two-lane road to board a Tippecanoe Valley School Corp. bus that had stopped with its stop-arm deployed, Slocum said.

CBS affiliate WTTV reports the truck was going in the opposite direction, according to the state police. No students were on the bus at the time of the crash.

Slocum didn't say which school the children attend. Police are interviewing the woman who was driving the pickup.

Slocum called the accident a "terrible tragedy."