Agree with the comments re: the audio. Nothing pleasant there, however it does provide relatively good insight for an SA analysis
We've had strong westerly winds the past few days so runway 30 was in use which is not the predominant operation at FDK. Controller was working 2 helis in the pattern, 2 planes doing practice instrument approaches to 23, cleared the accident heli (3rd heli) for takeoff from "A" for pattern work, told the accident airplane to report 3 miles west and later cleared him to land with the provision to maintain his altitude until turning base, then began issuing an IFR clearance to departing jet. That's a pretty signficant work load especially for a contract tower controller. I've flown in that controllers airspace and have been favorably impressed with her SA.
Clearly she expected there to be vertical separation as she believed the helis to be at 1000MSL (~700 AGL). Published TPA for a Cirrus is 1000' AGL (~1300 MSL). I never heard a altimeter setting given to the arriving Cirrus, but just a bit of slop in setting the altimiter in each aircraft plus a bit of distraction or fatigue can easily erase 300' of seperation. The "I've got 2 of them in sight" comment is absoluetly chilling.
I think its doubtful there will be radar tracks that low to allow a forensic examination of the aircraft tracks.
Edited to add, This accident doesn't give me a warm and fuzzy feeling about the proposed camera based remote tower experiment proposed for Leesburg (Just south of FDK). I've always maintained its a fallacy that towers make airports "safer" just by their existance. At airports like FDK and JYO where the traffic mix is heavily biased towards VFR operations I feel there is minimal to negative impact on safety by having a tower.