Authorities on Sunday declined to speculate on what caused a medical evacuation jet to crash off the coast of Nantucket on Saturday morning, as crews continue to search for the six missing passengers.

Rear Adm. Matt Lake, commander of the Coast Guard’s Northeast District, said at a news conference Sunday that Coast Guard crews, Massachusetts authorities and the New York National Guard’s air rescue wing have searched roughly 2,840 square miles over more than 53 hours.

“This is a large and demanding search area. Our crews are employing fixed-wing aircraft, helicopters, boats and cutters to search the area,” Lake said. He added that authorities hope to use sonar to search the seabed.

He declined to say whether the mission had shifted from rescue to recovery, which would mean no survivors are expected. He also declined to provide the passengers’ names or ages “out of respect to the families and next of kin.”

The U.S. Coast Guard launched a search Saturday morning and found a debris field approximately 20 miles off the coast of Massachusetts’ Nantucket island later that day.

Air traffic controllers lost contact with the jet early Saturday as it flew from Bermuda to Boston. One controller asked the crew whether it was declaring an emergency and “coming in blind,” according to audio obtained by NBC News.

The six people on board were four Canadians and two Bermudians. A source familiar with the matter told NBC News the Bermudians were a patient and the patient’s son.

The Bermudian government said Saturday that it was in contact with the families of those on board. The Coast Guard said it had notified next of kin.

The flight was scheduled to last less than two hours, according to data from FlightAware, a site that tracks flights in real time. The jet took off from L.F. Wade International Airport in Bermuda at about 12:40 a.m. and was due to land at Boston Logan International Airport just before 1:30 a.m. Bermuda is a British overseas territory in the North Atlantic, about 775 miles from Boston.

At around 1:05 a.m., FlightAware shows the jet dropping from 24,000 feet to 11,000 feet over the Atlantic Ocean.

The plane was operated by Latitude Air Ambulance, an Ontario, Canada-based medical evacuation company that arranges private flights for sick or injured patients. According to the company’s website, a typical flight is staffed by two pilots and two medical crew members and carries a patient and an escort. The Gulfstream jet that crashed Saturday was equipped to carry full ICU medical equipment and supplies.

Lake said Sunday that investigators would likely try to locate the plane’s flight recorder system, or “black box,” which is designed to preserve evidence after a crash.

“There’s no indications that we found a black box. But again, it’s very difficult because that debris is extensive,” Lake said.