ROCHESTER, Ind. (CBSDFW.COM/AP) — It was a tragedy that brought even first responders to tears. A 9-year-old girl and her twin 6-year-old brothers were struck and killed by a pickup truck as they crossed a northern Indiana road to board a school bus before sunrise Tuesday, police said. A fourth child was critically injured and airlifted to a hospital.

A Tippecanoe Valley School Corp. bus had stopped and lowered its stop arm around 7 a.m. on the road near Rochester just before a northbound pickup truck slammed into the children as they crossed the southbound lane, Indiana State Police Sgt. Tony Slocum said.

Six-year-old twin brothers Xzavier and Mason Ingle and their 9-year-old sister, Alivia Stahl, died at the scene, he said. The rural Rochester residents were students at nearby Mentone Elementary School.

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Alivia Stahl (middle) with her twin brothers Xzavier and Mason Ingle. (Family Photo)

Slocum said the pickup’s driver, Alyssa Shepherd, 24, of Rochester, was arrested and charged with three counts of reckless homicide and one misdemeanor count of passing a school bus when arm signal device is extended, causing bodily injury. Shepherd was being held at the Fulton County Jail on a $15,000 surety bond.

“Troopers and first responders don’t do a lot of crying at scenes like this but today there were some tears shed,” Slocum said.

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Alyssa Shepherd mugshot (Indiana State Police)

The great-aunt of the three siblings who were killed, Pamela Pugh, told the South Bend Tribune that she was stunned and shaken by their deaths.

“I’m just trying to make sense of all of it. There are no words,” she said.

The uncle of the three children described Alivia as a “mother hen” to her little brothers.

“It’s horrible that this happened,” Elgin Ingle said. “They were holding hands every day on the way to the bus stop including today…it looks like Alivia stepped in front of the car for the boys. It looks like Alivia seen what was happening, and she stepped up like a hero. I wish she could’ve saved her brothers, but she’s still a hero.”

Slocum said an 11-year-old boy not related to the deceased siblings suffered multiple broken bones when he was also hit by the pickup. That child, Maverik Lowe, was in critical condition at a Fort Wayne hospital and was undergoing surgery.

The boy was conscious and speaking to emergency workers before he was airlifted to the hospital, Slocum said.

Slocum said the students had the right of way as they crossed the street from the rural mobile home community where they lived.

“It’s an unspeakable tragedy,” he said. “We all have a responsibility to share the roadway and we’re all responsible for making sure our children get to and from school safely.”

Detectives were interviewing witnesses and a crash reconstruction team was at the scene near Rochester, about 100 miles north of Indianapolis.

The National Transportation Safety Board announced on Twitter that it was sending a team of investigators to the scene that should arrive Wednesday.

The Tippecanoe Valley School Corp. said in a statement posted on Facebook that counselors had been dispatched to its schools to help students, staff and parents. It urged “the community to come together to pray for the families.”

Gov. Eric Holcomb said in a statement that he and his wife, Janet, were mourning the students’ deaths. He urged Indiana residents to send their “deepest prayers for the strength needed to endure such a time.”

“Words cannot express the depth of sorrow Janet and I feel, which only pales in comparison to what family, friends, teachers, classmates and community are feeling right now,” Holcomb said.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)