According to EU border officials, Greece ignored an offer to send a plane to watch over a migrant boat that later sank with a significant loss of life.
The incident last week claimed the lives of at least 82 people, but the UN estimates that 500 more may have drowned.
Greece has come under fire for not responding to the disaster more effectively.
The BBC also discovered that, contrary to the Greek assertion that it was on a safe, steady course, the migrant boat made very little movement in the hours before it capsized.
The crowded fishing boat had departed from Libya when it was first spotted early on June 13 in international waters heading toward Greece.
A plane operated by Frontex, the EU's border agency, spotted it and had to refuel.
Frontex asserts that it made the Greek coast guard aware of its offer to send the plane back to the fishing boat to keep an eye on the situation, but they did not respond.
According to the Greek government, those onboard told the coastguards they wanted to be left alone so they could travel to Italy, refuting claims that they failed to react to the tragedy quickly enough.
The BBC's analysis of other ships' movements on the disaster day, however, strongly suggests that the ship was barely moving for at least seven hours prior to its capsize, which is in direct opposition to the official account.
Regarding this most recent allegation that it ignored Frontex's offer of additional aerial assistance, the Greek coastguard has not responded.
Approximately 80 kilometers (50 miles) south-west of the coastal town of Pylos, according to officials, the boat went down on June 14 after 02:30 local time.
More than 100 people were saved, but according to survivors, there may have been up to 750 people on the boat, including about 100 kids in the hold.
Rana Sanaullah, Pakistan's interior minister, stated that at least 350 Pakistanis were on board and said that "perhaps there has never been such a large toll in any incident before, even in terrorist incidents.".
Syrians and Egyptians are among those who are feared dead.
Nine Egyptian men were accused of exposing lives to danger, negligent manslaughter, causing a shipwreck, and human trafficking. They appeared in court in the Greek city of Kalamata on Monday.
Each entered a not-guilty plea.
