Naval Service divers will use side-scan sonar as they attempt to locate wreckage to help solve the mystery of the downed Irish Coast Guard helicopter Rescue 116.
The families of three missing crew are praying for their recovery, after a tragedy that left a "dark cloud over Ireland".
A full-scale search will continue this morning. One crew member, Captain Dara Fitzpatrick (45), was recovered but was later pronounced dead. The missing men are Ciaran Smith, Mark Duffy and Paul Ormsby.
Coast Guard sources said there was no evidence that the crew of the stricken helicopter had attempted to send a mayday call.
This would suggest that whatever happened, the crash was unlikely to have been caused by a mechanical fault. But the sources stressed that it was too early in the investigation to speculate on the cause.