This comment was written by a US Coast Guard helicopter pilot. It gives a lot of information on how the two aircraft should have been able to share the space and some speculation on what went wrong:
A helicopter instructor suggests that possibly the helicopter pilots, who were told to go behind an aircraft that was landing, were looking at the previous aircraft that was landing.[1] That's just speculation at this point.
Helicopter was on Helicopter Route 4, per the map, apparently on course.
Aircraft was on approach to Runway 33, apparently on course.
That helicopter route crosses the approach to runway 33.
That's controlled airspace. How did they both have clearance to be there?
We'll know more tomorrow as all the audio and radar recordings are examined.