State police are investigating

as homicide by vehicle, according to documents filed in the case on Tuesday.

Luke Lee Frederick Stump Jr., 5, and his 2-year-old twin siblings, Megyn Diane Stump and Troy Matthew Stump, all of Duncannon, were pronounced dead at the scene Saturday night.

The driver, Laurie Hatfield, 47, the children’s grandmother,

.

Hatfield, who has a Duncannon address,

, troopers said in an affidavit of probable cause seeking a warrant to search Hatfield’s 2001 Ford Taurus station wagon. They were looking for evidence of controlled-substance or medication use, car defects, data-collection devices, paint chips, and hair, blood and tissue samples, troopers said.

The driver in that crash identified Hatfield and her car as having hit his or her vehicle about 2 miles from the fatal crash scene, documents state.

That driver told investigators false information was given to him or her and that the car sped off as he or she tried to write down the license plate number, troopers said. The driver recorded the license plate as ENE1248; Hatfield’s license plate is ENE1268, troopers said.

Troopers interviewed Hatfield on Monday in the hospital, and she told them the road and weather conditions were clear, documents state. She described the trip as “smooth sailing,” and that there were no problems or issues with the car, according to the documents.

Hatfield told emergency medical personnel at the scene that she could not remember what happened, but that she was not drinking but does take several prescription medications, troopers said.

Her son, Luke Lee Frederick Stump, the children’s father, told investigators on Monday that his mother takes several medications for treatment of cancer and thyroid, heart and kidney problems, troopers said. Hatfield also takes daily pain pills that she would have had with her at the time of the crash, Stump told troopers.

Troopers are investigating whether the children were restrained in car seats in the crash.

At least one of them was not in a child safety seat, Dauphin County District Attorney Edward M. Marsico said.

State law calls for a child younger than 4 to be in a child-safety seat, and children age 4 to 8 must be restrained in booster seats. Even if the evidence shows that all three children were not restrained, it cannot be used to prosecute Hatfield for vehicular homicide or other violations except failing to properly restrain the children, Marsico said.

“The law says that is not admissible in criminal proceedings,” Marsico said.

Hatfield’s car left the highway, went over a guardrail and struck trees and bushes before landing on its roof 25 feet down an embankment on the Susquehanna River side of the highway near Route 325, or Mountain Road, troopers said.

Hatfield was charged in 2005 by the state attorney general’s office with two counts of acquiring or obtaining possession of controlled substances by misrepresentation. Those charges were dropped in March 2007 after she completed a year of probation under the accelerated rehabilitation disposition program, court records show.

Records also show Hatfield pleaded guilty to bad check charges in five cases from January 2008 to November 2009.