Two young girls died and four of their family members, including an infant sister, were injured in a fast-moving fire that destroyed a Myersville, Md., home late Thursday, according to authorities.

Sophie P. Lillard, 6, and Madigan G. Lillard, 3, died of smoke inhalation, according to the Frederick County sheriff’s office.

Autopsies performed Friday at the Office of the Chief Medical Examiner in Baltimore determined that their deaths were accidental, the sheriff’s office said in a news release.

The victims’ father, Jack R. Lillard, 39, was flown by helicopter to MedStar Washington Hospital Center. A sister, Morgan B. Lillard, 8, was taken to Children’s Hospital, according to Cpl. Jennifer Bailey, a spokeswoman for the sheriff’s office. Their conditions were unavailable early Friday afternoon.

Angela C. Lillard, 33, and her daughter, 7-month-old Sadie Lillard, were transported to Frederick Memorial Hospital, where they both were treated and later released, Bailey said.

The cause of the fire, which is estimated to have caused $200,000 in damage, is under investigation, she said.

The two-alarm blaze was reported about 11:20 p.m. at the house in the 3900 block of Highland Avenue. When firefighters arrived, they found the home already engulfed in flames.

Firefighters were told that two girls were still inside, but they were unable to find the children because of the intensity of the blaze and the damage to the unstable structure, Bailey said.

About 40 units and 75 firefighters from companies in Frederick and Washington counties, as well as Fort Detrick, responded to the scene, Bailey said. It took 90 minutes to bring the blaze under control.

A firefighter from Middletown Volunteer Fire Company also was taken to Frederick Memorial Hospital, but no details on his injuries were released by the fire company, Bailey said.

Gazette staff writer Tripp Laino contributed to this report.