Greece did not respond to an offer to send a plane to monitor a migrant boat that later sank with huge loss of life, EU border officials say.
At least 82 people are known to have died in the incident last week, but the UN says a further 500 may have drowned.
Greece has faced criticism for not doing more to respond to the disaster.
The BBC also found that the migrant boat hardly moved in the hours before it capsized contradicting the Greek claim it was on a safe, steady course.
Offer not responded to, Frontex says
The overcrowded fishing boat had set out from Libya and was first detected early in the morning of 13 June in international waters moving towards Greece.
It was spotted by a plane operated by Frontex, the EU’s border agency, which then needed to refuel.
Frontex claims it offered to send the plane back to the fishing boat to monitor the situation but that the Greek coast guard never replied.
The Greek authorities have rejected claims they didn’t act quickly enough to the unfolding tragedy, insisting those onboard told coastguards they wanted to be left alone so they could travel to Italy.
But BBC analysis of the movement of other ships on the day of the disaster strongly suggests the vessel was hardly moving for at least seven hours before it capsized – contradicting the official account.
The Greek coastguard has not commented on this latest claim that it did not respond to the offer of further aerial help from Frontex.
Officials have said the boat went down around 80km (50 miles) south-west of the coastal town Pylos after 02:04 on 14 June local time.
More than 100 people were rescued, but survivors have estimated that as many as 750 people may have been on the boat, including around 100 children in the hold.
Pakistan’s interior minister, Rana Sanaullah, has said that at least 350 Pakistanis were onboard, adding that “perhaps there has never been such a large toll in any incident before, even in terrorist incidents”.
Egyptians and Syrians are also among those feared dead.
On Monday, nine Egyptian men appeared in court in the Greek city of Kalamata to face charges of negligent manslaughter, exposing lives to danger, causing a shipwreck and human trafficking.
All pleaded not guilty.