Three people were killed when a medical helicopter crashed in Kentucky while returning to base after transporting a patient to the hospital in Laurel County, authorities told NBC on Friday.

The cause of the crash, which occurred in Clay County on Thursday night, is yet unknown. But witnesses believe dense fog in the area might have resulted in the accident.

The aircraft crashed in the parking lot of an elementary school at around 11.30 p.m. No one on the ground was reported hurt and the school property was not damaged although power lines were hit.

The helicopter company, Air Evac Lifeteam, said the Federal Aviation Administration and the National Transportation Safety Board, or NTSB, have been notified and an investigation will soon begin.

In a statement on its Facebook page, Air Evac Lifeteam wrote that they are devastated at the lost of its three members -- pilot Eddie Sizemore, flight paramedic Herman "Lee" Dobbs and flight nurse Jesse Jones, Fox News reported.

“We are saddened to confirm that three members of our Manchester, Ky., crew were killed in a helicopter accident late Thursday evening. We are devastated at the loss of these crew members who we consider family. We have no details regarding the cause of the accident but will be working with the NTSB in coming days as they conduct their investigation,” the company's Facebook page said.