London (CNN)A pair of skydivers narrowly avoided colliding with two US fighter planes traveling at almost 350 mph over the UK, a report has revealed.
The parachutists, who have not been identified, were in free fall at around 120 mph when the two F-15 jets passed beneath them on April 17.
The near miss -- or "airprox" as it is officially referred to -- was captured on a GoPro camera attached to the helmet of one of the skydivers, according to a report by the UK Airprox Board, which monitors such incidents. The parachutist assessed the risk of collision as "medium," the report's authors said.
The skydivers had taken off from Chatteris airfield in Cambridgeshire, while the jets set out from the Royal Air Force (RAF) base in Lakenheath, Suffolk, home to the US Air Force's 48th Fighter Wing. Weather conditions were good and the report stated that the skydivers had 10 kilometers visibility, while the pilots had a clear view for 50 kilometers.
Designated parachute drop zones must be marked on aviation charts, which means it is incumbent on aircraft operators in the area to know that skydivers could be in flight. According to the report, the RAF base received a call from Chatteris each morning to inform them of any activity.
One of the pilots told investigators that he was not aware that Chatteris was open to skydivers at that time and nothing had been mentioned by air traffic control (ATC). This should not have been the case, according to the report, and steps have since been taken to prevent further incidents.
The report, which was published in recent weeks, states: "When asked whether the pilots should be aware of Chatteris, the Board were told that it was included in their arrival training at Lakenheath, and that, as a result of this incident, the unit was re-briefing all of its crews to ensure they were all aware of its status."
There were several contributing factors, according to the report, which suggested that the breakdown in communication may have partly come about due to the heavy workload of Lakenheath air traffic control.
It stated: "The Lakenheath controller was aware of the position of Chatteris, and the Board were told that each morning Chatteris called ATC to advise when they are active; the paradropping aircraft also calls on the frequency when they are airborne with parachutists.
"The controller believed he would have plenty of time to change the heading of the F-15s when they came on frequency. However, the frequency became busy just as they transferred and so, by the time the F-15 pilots checked in with the controller, they were already about to fly over Chatteris."
Once the skydivers spotted the jets, "there was very little they could do to avoid the situation, having no control over their speed or direction whilst in free-fall", though some members of the board questioned whether they could have slowed themselves down by opening their parachutes.
While the skydivers have not been identified, the fact that they did not open their parachutes and were not traveling in tandem may suggest that they had considerable experience in the extreme sport.
The report described it as "unfortunate" that the air traffic controller at Lakenheath "had not realized the F-15s were heading towards Chatteris."
Investigators were "unable to reliably determine how close the F-15s had been to the parachutists from the video but the F-15s were clearly distinguishable," raising the question of how high the risk of collision was.
Nick Wall, an independent aviation safety expert, told CNN that the category B assigned to the incident is considered "pretty serious."
But, he, said, the close call had come about due to an "unfortunate" combination of factors, including the speed of the fighter jets and the pilots' inability to communicate with air traffic control because of the busy frequency.
"At the speed that they were traveling at, distance passes by very quickly. So by the time they did get through they were passing over Chatteris, which is why it was an unfortunate set of circumstances."
He added: "The US Air Force has taken the lesson on board and have taken action to do something about it."
Col. Will Marshall, 48th Fighter Wing commander at Lakenheath, said in a statement to CNN: "UK airspace is incredibly complex and often congested, and the safety of our aircrew as well as those we share the skies with is our number one priority.
"We are using this incident to reinforce the vital importance of situational awareness and attention to detail for our all of our air traffic controllers and aircrew."
Skydive Chatteris Club refused to comment on the incident when approached by CNN.