On the night of June 14, Captain Richard Kirkby is piloting the Mayan Queen IV, a luxury yacht belonging to a Mexican multibillionaire, through the calm, black waters of the Mediterranean when he receives an emergency call. "Ship sinking. Large number of people. Vessels in the vicinity are requested to initiate search and rescue operations." The crew hears the screams from people drowning before they can see them.

The shipwreck that takes place that night would turn out to be the deadliest in the Mediterranean in many years. Around 750 people are thought to have been on board the fishing boat that went down off the coast of the Peloponnese. When the Mayan Queen IV reaches the site at 2:55 a.m., only the lights of another ship can be seen. They belong to the Greek Coast Guard, vessel LS 920 – according to investigation files that DER SPIEGEL and its partners have acquired.

But the Greeks cannot be reached via radio. So three crew members from the Mayan Queen IV climb into a life boat and start searching for survivors, constantly heading toward the cries for help. They stay as quiet as they can so as not to miss a single voice. Ultimately, they will pull 15 people out of the water.

Early in the morning, the Greek Coast Guard requests permission to bring additional survivors on board. The Greek vessel is too small to safely bring all the survivors to shore. But the Mayan Queen IV – a ship with four decks, tinted windows and a helicopter landing pad – is large enough. At 7:20 a.m., the yacht sets course for Kalamata. On board are 100 of a total of 104 survivors – migrants wrapped in silver emergency blankets cowering where the super-rich are normally sunning themselves.

Hundreds of refugees don’t survive this night – despite the fact that the Greek Coast Guard arrived at the site several hours before the accident. As early as the morning of the previous day, an Italian agency had sent them a warning and a non-governmental organization had forwarded an SOS from the fishing boat. Even the European Union border control agency Frontex had identified the ship’s plight and offered additional assistance. How can it be that hundreds of migrants died anyway? It is a question that has plagued the Greek Coast Guard for the last two weeks.

The accusations that survivors have leveled at the Greeks are serious: Did the Coast Guard leave the people to their fate for too long? Were they trying to pull the ship into Italian waters – as some testimony seems to indicate? Perhaps to keep hundreds of migrants from landing in Greece?

A team of reporters from DER SPIEGEL joined forces with the nonprofit newsroom Lighthouse Reports, investigative journalism consortium Reporters United, the Spanish newspaper El País, the Syrian investigative reporting outlet Siraj and the German public broadcaster ARD to explore these questions. The reporters interviewed survivors, many of whom had already turned to the aid organization Consolidated Rescue Group. They examined leaked investigative reports, videos and geodata and spoke with sources inside Frontex.

The reporting indicates that, at the very least, the Greek Coast Guard may have made grave errors. Sixteen refugees have accused the Greeks, for example, of causing the fishing boat to capsize, while seven are convinced that Greek rescue attempts were hesitant at best – which would mean they were willing to accept the deaths of hundreds of people. There are also serious doubts about the willingness of Greek authorities to thoroughly investigate the disaster. The leaked investigation reports raise questions as to whether Greek officials may have altered testimony in their favor.

One of those who survived, we’ll call him Manhal Abdulkareem, tells his story in mid-June from the Greek camp Malakasa. He requests that we not use his real name or even describe him out of fear of how the Greek authorities might react. What he has to say does not paint them in a positive light.

The Syrian once worked as a stonemason in Jordan. Last spring, he decided to risk the crossing to Italy. He traveled to Libya and boarded the vessel in the port city of Tobruk on June 9. Abdulkareem is one of hundreds of people who crowded onto the vessel, and he was one of the lucky ones: He was able to buy himself a place on deck. Later, it would save his life.

Other refugees crowded into the boat’s cold storage room. According to survivors, women and children were below decks, many of them from Pakistan. For them, the belly of the ship would turn into a coffin.

Abdulkareem’s account of the initial days onboard the ship is consistent with the stories told by other survivors. He says that they began running out of water on the third of five days onboard, that the motor cut out on several occasions and that the captain seemed to have lost his orientation. The goal of reaching Italy was more distant than ever.

The Greek Coast Guard was also aware of the dire situation onboard the fishing boat. On the morning of June 13, they received the first warning from the Italian Coast Guard. Frontex agents filmed the ship from the air at midday. At 5:13 p.m. local time, the non-governmental organization Alarmphone wrote an email to the Greek authorities. The email noted that there were 750 people on the ship. "They are requesting urgent assistance."

At the time of the call for help, the fishing vessel was around 80 kilometers (50 miles) off the coast of the Peloponnese. Nevertheless, the Greek Coast Guard sent a ship that was anchored in far-away Crete.

At least two freighters supplied the fishing vessel with water, but they didn’t take anyone onboard. Abdulkareem and other survivors say that by this point, two passengers on the boat had already died. The Greek Coast Guard ship only arrived at 10:40 p.m.

There are two versions for what then took place.

Manhal Abdulkareem reports that the Greek Coast Guard escorted their ship for a time, until the fishing boat’s engine again cut out. Then, he says, the Coast Guard attached a rope to the vessel. "We thought they knew what they were doing," says Abdulkareem.

The Coast Guard, he says, towed the vessel at a rapid speed, first to the right, then the left, and then back to the right – and then it capsized. Fifteen additional survivors tell a similar story. Some believe the behavior of the Coast Guard was accidental. Others think it was intentional.

When the vessel capsized, there were people trapped inside its hull. One survivor says he heard them knocking. Those who were on deck jumped into the water. "People were falling on us," says one man from Egypt. Some clung to the sinking vessel, while others grabbed in a panic for anything that was floating, including other people.

"I know how to swim, but that wasn’t enough," Abdulkareem would later say. He says he had to avoid others so that he wouldn’t be pulled down into the depths. Four survivors say that the Coast Guard put those in the water in even greater danger by maneuvering in such a way that created large waves.

While still in the water, Abdulkareem began searching for his brother, but was unable to find him. As the vessel was sinking, say survivors, the Greek Coast Guard ship pulled back to a distance of hundreds of meters.

Abdulkareem and six others accuse the Greeks of delayed rescue efforts and only launching inflatable dinghies after significant time had passed. Some estimate that several minutes passed before they took any action at all. Others say the delay was fully half an hour. "They could have saved many people," says a survivor from Syria. Abdulkareem’s brother still hasn’t been found.

The Greek Coast Guard has a competing account for what took place. According to an official log from June 14, their ship reported on the evening prior to the disaster that the refugees were "on a stable course" – a claim that video evidence and tracking data refute. The people on board, according to the official account, rejected assistance because they "wanted nothing more than to continue onward to Italy." If the Greek Coast Guard is to be believed, the fishing boat capsized shortly after 2 a.m. The first official log provides no cause for the accident.

Later, the Greek government spokesman said that the Coast Guard had attached a rope to the boat. But only to "stabilize" the vessel. By the time of the accident, the rope had already been cast off, the spokesman said, and the fishing vessel had never been towed. The rope, he insists, was not the cause of the shipwreck. In an interview with CNN, a Coast Guard spokesman speculated that panic may have broken out onboard, leading to the boat listing to one side.

There is no proof for either version. But doubts about the Greek account are significant, even within Frontex. At the agency’s headquarters in Warsaw, EU border guards can follow in real time what is taking place on the EU’s external borders. In this case, the agents must have realized early on the danger that the migrants were in.

On two occasions – at 6:35 p.m. and at 9:34 p.m. – they offered to send the airplane back to the ship that the migrants had already seen at midday. It was refueled and ready to take off, according to an internal memo that DER SPIEGEL has obtained. But the Greek Rescue Coordination Center in Piraeus, Frontex says, ignored the offer. The plane remained on the ground.

The only other available aircraft, a Frontex drone, was initially sent to another distress call, according to Frontex. It only arrived at the scene after the fishing vessel had sunk. In Brussels, hardly anyone believes that the rebuff of Frontex was an accident. Many see a pattern: Greek authorities systematically send away Frontex units, says one Brussels official. That happens particularly often, the official says, in situations that later turn out to be controversial.

The mistrust with which Athens now finds itself confronted – even from EU institutions – has a lot to do with previous violations of international law on the Aegean. The Greek Coast Guard has repeatedly towed groups of refugees back into Turkish waters – before then abandoning them on life rafts with no means of propulsion.

Proof for such pushbacks has become so overwhelming that the Frontex fundamental rights officer recently recommended that the organization suspend cooperation with the Greek Coast Guard. The "strongest possible measures" are necessary to ensure that the Greeks once again begin complying with applicable law, reads an internal memo that DER SPIEGEL has obtained. Joint missions can only be resumed once a new basis for trust has been established, the memo continues.

The skepticism has become so great that Frontex has even sent a team to Greece to question survivors itself. Two Frontex officials say that the results of investigations conducted thus far seem to contradict the Greek version of events. One Greek lawyer is even demanding an official state investigation of the Coast Guard for manslaughter through failure to render aid.

Most survivors, though, don’t believe that the Greek state will investigate the role played by its own Coast Guard. The treatment they received in the days following the catastrophe was too poor for such optimism.

Sami Al Yafi, a young Syrian, is one of them. He, too, has asked that his real name not be printed out of fear of the Greek authorities. He accuses the Coast Guard of manipulating his statement. He claims to have clearly testified that the Coast Guard had caused the ship to capsize, but he was unable to find that statement in the transcript of his interview. An additional survivor says that he had a similar experience.

There are also corresponding inconsistencies in the investigation file. In six instances, according to the file, survivors said nothing about a tow rope in their first interview with the Coast Guard – or at least there is no mention of such in the minutes taken by the Coast Guard. Later, in interviews with public prosecutors, they then accused the Coast Guard of causing the capsizing by towing the vessel.

Moreover, the minutes taken by the Greek Coast Guard frequently include the exact same formulations. According to those minutes, four survivors used exactly the same words in describing the events – despite the fact that the interviews were led by different interpreters. In one case, a member of the Coast Guard apparently acted as an interpreter.