The National Transportation Safety Board says a witness reported seeing a flash in the sky before a helicopter crashed near an Oklahoma City nursing home last month, killing two people.

It’s not clear what could have caused such a flash.

The preliminary report, released Wednesday, does not spell out why the helicopter crashed on Feb. 22, killing the pilot and a flight nurse. The third person onboard was seriously injured.

Investigators say satellite data showed the helicopter took off from a helipad at Integris-Baptist Medical Center in Oklahoma City, but that the data stopped less than four minutes into the flight.

The report says the helicopter was headed to Watonga, about 70 miles away, to transfer a cardiac patient to the University of Oklahoma Medical Center.