Dempsey's daughter, Bethany, and granddaughter, Lauren died in the August 27 crash, along with 7-year-old Abigail Cullen. Cullen's father, Michael, remains in the hospital. Michael Cullen's son, Michael Jr., was hospitalized but has been released.
The accident on Route 3 remains under investigation, but police said a flatbed truck failed to slow down as it exited Route 32 westbound, striking a 2009 Camry and setting off a chain reaction of collisions.
In his letter, Sheriff Dempsey describes the agony of notifying family members and friends of the accident, but also describes an overwhelming number of "phone calls, cards, flowers, food, visits and hugs." He wrote that an estimated 1,400 people came to Bethany and Lauren Dempsey's funeral.
"We are humbled and blessed," Dempsey wrote on Facebook.
A memorial service for Abigail Cullen is scheduled for Thursday in Fredericksburg.