Three young children from Maryland and a family friend died in a car crash in South Carolina on Friday as they headed to a wedding.

Three members of the Harley family of Wheaton, Maryland, were killed when a tractor-trailer slammed into the car they were riding in: Devin Jr., 7; Nicholas, 6; and Leah, 5. Family friend Lani Tran, a 27-year-old from Silver Spring, Maryland, also was killed.

The children's mother, Nancy Harley, was driving a 2001 Ford Taurus on I-95 in Colleton County, South Carolina with her husband, Devin Harley, in the passenger seat. The children and Tran sat in the backseat.

About 4 a.m. Friday, a tractor-trailer slammed into the back of the car.

The huge vehicle ran over the car with the family inside. Tran and the children were killed, and their parents were rushed to a hospital.

"I don't think we're ever going to recover from this," said Amy Moscoso. She was the children's cousin.

"They went too early. They were the best kids ever," she said.

All three children were students at Arcola Elementary School in Silver Spring.

"We are recovering from a news that is not imaginable," school principal Emanuel Jean-Phillipe said.

The children's grandfather spoke with News4 about the tragedy as he drove home to the D.C. area.

"They were three beautiful kids, three beautiful, innocent children. This is the worst thing that could ever happen to me and my family," Charles Cain said.

Relatives said that before the crash, the Harleys were having car trouble. They were in the right-hand lane with flashers on, trying to get to a safe place, when the tractor-trailer hit them from behind.

Devin Harley is said to be OK physically. Nancy Harley is said to be in a coma. Family members said she did not know what had happened.

GoFundMe pages been set up to pay for the costs of the children's funerals and their parents' medical expenses.