Greek police tried to capture the suspected ringleader of the Paris terror attacks, Abdelhamid Abaaoud, in January but the operation failed.
A Belgian anti-terrorism source told the BBC the Athens operation planned to target Abaaoud before anti-terror raids in Belgium, but that did not happen.
Abaaoud had been directing the Belgian cell by phone from Athens.
Abaaoud died in a battle with French police five days after the 13 November Paris attacks that killed 130 people.
The Greek operation was supposed to have taken place before the one carried out by security forces in Verviers, eastern Belgium, on 15 January. That raid saw an exchange of fire that left two suspected jihadists dead.
Greek authorities were on Abaaoud’s trail, believing him to be running the Belgian cell by mobile phone from Athens.
Anti-terror sources told that that a senior Belgian police officer was in Athens co-ordinating the hunt for Abaaoud with his Greek counterparts before the raid on the Verviers cell.
It remains unclear why or how Abaaoud slipped through the Greek net. There may have been an attempt to track him down to a city centre square by tracing the signal of his mobile phone. But that did not work.
The Greek authorities are not confirming any details – all that is known is that he got away.
Greek police only carried out raids in Athens two days after Verviers, on 17 January.
Earlier that day Belgian media had reported that authorities there were seeking Abaaoud, a Brussels resident of Moroccan origin, who was believed to be in hiding in Greece.
Greek police raided two flats in Athens.
One Algerian man was eventually extradited to Belgium but Abaaoud was not to be found.
It is now known that traces of DNA recovered in both flats match samples recovered from Abaaoud’s body in Paris.
A neighbour at one of the flats, Vasilis Katsanos, said he had seen Abdelhamid Abaaoud in the street outside on at least two occasions.
Abdelhamid Abaaoud had been implicated in four out of six foiled attacks since this spring in France and sentenced to 20 years in prison in absentia, French Interior Minister Bernard Cazeneuve has said.
Abaaoud is not the only link between Greece and the Paris attacks.
Salah Abdeslam – who is still on the run – travelled to Greece by ferry from Italy on 1 August, leaving three days later.
And two of the suicide bombers who attacked the Stade de France crossed by boat from Turkey to the island of Leros in October, posing as refugees.
Much of the detail that has emerged in Athens raises questions about how to create a better exchange of information and closer cooperation between anti-terrorism authorities in different European countries.
But the link with Abaaoud is also a what-might-have-been.
If he had been captured in Athens in January, the attacks in Paris might never have taken place.