Police are using sonar equipment to try and find the boat two men were in before they went missing on a fishing trip.
David Archibald, 59, and Andrew Conlon, 56, from Dalkeith had been on a regular Sunday fishing trip at Gladhouse Reservoir, Midlothian.
When they did not return to their homes as darkness fell, the pair were reported missing.
Lothian and Borders Police launched a search operation and called in a RAF helicopter for help. The men's bodies were found in the reservoir in the early hours of Monday morning.
Neither of the men was wearing lifejackets when they were out on the water.
Mr Conlon is listed at the Fishing Manager at the reservoir, with fishermen advised to call him if they are unsure about the weather conditions.
Lothian and Borders Police need to find the boat the men were in to try and work out what happened.
The sonar equipment allows them to search below the surface of the water for the boat.
A post-mortem examination will be carried out and a report submitted to the procurator fiscal.
Inspector Neil Simpson of Lothian and Borders Police said: "We are carrying out extensive inquiries to find out what happened to these two men. They had been taking part in what was a routine activity for them when something happened, which resulted in them losing their lives.
"Early indications are that this was a tragic accident and there are no suspicious circumstances.
"Our thoughts are with the two families and we have officers working with them to provide support. They are being kept fully informed."