The St. Mary's County sheriff's office says the buggy moved into the intersection during a green light but was still in the crossing when the light turned red.
One of the buggy's occupants, 12-year-old Saloma Kathleen Stoltzfus, died of her injuries
Deputies say the pickup's driver didn't notice the horse-drawn vehicle, which had three people inside. The resulting collision turned the buggy on its side. The other two occupants were ejected from the buggy when the truck slammed into it, according to CBS affiliate WUSA. They were treated and released from the hospital.
The investigation continues, and the truck driver hasn't been charged.