National Transportation Safety Board investigators probing the horrific collision that left three Indiana schoolchildren dead at a bus stop in October say they are looking at two similar incidents in Georgia and Mississippi to help determine what went wrong.

In a preliminary report of the Fulton County collision Wednesday, the NTSB said twin brothers Xzavier and Mason Ingle, 6, and Alivia Stahl, 9, their sister, were hit on a “two-lane highway with a posted speed limit of 55 mph” in Rochester while trying to board the bus – the same rate as the other two incidents.

“All aspects of the Rochester, Indiana, crash remain under investigation as the NTSB focuses on determining the probable cause, with the intent of issuing safety recommendations to prevent similar crashes,” the NTSB said.

The 24-year-old woman accused of striking the children, Alyssa Shepherd, has already pleaded not guilty to three counts of reckless homicide. She also faces a misdemeanor charge of passing a school bus when an arm signal device was extended.

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A prosecutor revealed Tuesday that Shepherd is expected to face a fifth charge for striking Maverik Lowe, who survived the collision. Shepherd could face up to 21 years in prison if convicted on all counts and her trial may begin as early as this summer if prosecutors and her legal team don't reach a plea deal, according to WNDU.

The two other incidents being looked at by the NTSB also happened in October last year.

In one case, a 10-year-old boy was killed and a 7-year-old boy was seriously injured after being struck by a car while trying to board a school bus in the early morning darkness in Hartsfield, Georgia, the NTSB says.

In the other case, a 9-year-old boy was killed after being hit by a pickup truck while crossing a highway to get onboard a bus in Baldwyn, Mississippi.

Both school bus operators had their stop arms deployed and warning lights on when the tragic collisions occurred.

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The family of those killed in the Indiana crash are now pushing for state lawmakers to pass a Senate bill that would heighten penalties for drivers who fail to stop for school buses and let districts reduce speed limits, according to Fox 59.

“At one point in time we were all on those buses, and now we’re here,” Michael Stahl, the father of Alivia, told the station. “So now it’s our responsibility to make sure that our children and the future is safe.”