The pilot of the ill-fated AgustaWestland AW109 that crashed in London on 16 January had requested to divert and land at the London Heliport in Battersea because of bad weather.
The light twin-engined helicopter was on a commercial flight from Redhill in Surrey to Elstree in Hertfordshire when it collided with a crane on top of a high-rise building at about 08:00 local time.
It came down on Wandsworth Road near Vauxhall train station, south of the river Thames, killing the pilot, the sole occupant of the aircraft, and one person on the ground. Thirteen others were injured and one remains in a critical condition.
Weather data from nearby London City airport at the time of the crash shows a ceiling of 100ft (30m) in conditions of freezing fog with visibility down to 600m.
On 7 January, the UK Civil Aviation Authority issued a NOTAM warning of a jib crane at a height of 770ft near London Heliport. It says the obstacle would be "lit at night".
The CAA says: "There are requirements for lighting on tall structures. In addition, where appropriate, very tall structures are also notified to pilots for flight-planning purposes, as was the case with the crane that was involved in this morning's accident."
London Heliport says it received a request from Heathrow air traffic control to allow the AW109 to divert to Battersea. It adds: "Earlier in the helicopter's journey the pilot had been receiving an air traffic control service from NATS. At no point in time were we able to establish contact with the helicopter and NATS was not providing [a]service [to it] at the time of the incident."
Kate Hoey, MP for Vauxhall, has called for an inquiry into the "increasing numbers of helicopters flying around London" among so many new high-rise buildings.
She says: "The river [Thames] is a kind of motorway for helicopters but the noise, of course, is horrendous sometimes, when we get a lot of helicopters hovering. Maybe we've come to take it almost for granted that people have the right to take their helicopter over London at any time and I think we may have to look at that."