PORTLAND – The pilot of a small plane that crashed off Maine’s coast, killing a couple from New Jersey, was relying on instruments to fly in cloudy weather, investigators said.

The crash on Sunday killed Edward and Maryann Mainardi, who were flying their 30-year-old Cessna 337 Skymaster from Millinocket, where they had a seasonal home, to Caldwell, N.J.

A National Transportation Safety Board investigation is in its initial stages. Luke Schiada of the NTSB said the probe would focus on the 73-year-old pilot’s health and experience, the plane’s maintenance history and performance and control tower records of weather conditions and radio communications.

The aircraft was demolished when it crashed into the Gulf of Maine about 18 miles east of Portland, and most of it sank in water 250 feet deep. The company that insures the aircraft will decide whether to recover the engine and other components, which the NTSB could analyze for mechanical problems.

The Mainardis’ remains have been turned over to the state medical examiner’s office for positive identification.

The Federal Aviation Administration said that while the plane was over Maine, the weather worsened and the pilot was unable to find clear visibility. He radioed the FAA control tower in Boston for permission to rely on his plane’s instruments to navigate.

Mainardi’s final communication was with the Brunswick Naval Air Station tower in Maine. In that exchange, he confirmed he would be monitored next by Portland’s tower and the new radio frequency, but never contacted the Portland tower, said FAA spokesman Jim Peters.

There was no indication in the FAA log of a distress call or problems on board. The plane was flying at about 8,000 feet when it disappeared from radar.

A preliminary NTSB report is expected by the end of next week and a more extensive report on the cause will take six to nine months.