How migrant tragedy unfolded on the high seas off Greece

As Greece confronts its worst sea disaster in years, questions are mounting about how potentially hundreds of migrants drowned even as their vessel was being shadowed by the Greek coastguard.

At least 78 people died when the fishing vessel flipped and capsized in international waters south-west of Greece early on Wednesday. Authorities rescued 104 people but hundreds are feared missing, with some witnesses saying up to 750 people were aboard.

According to timelines from authorities and activists, the Greek coastguard was alerted to the presence of the vessel 47 nautical miles (87 km) south west of Greece at around 0800 GMT on June 13. It capsized at around 2300 GMT that day.

Exactly what happened in the intervening 15 hours remains unclear. A migrant charity says a person they were in contact with on the boat said it was in distress, but Greek authorities say...