CAIRO — Egypt sent a submarine Sunday to join the hunt for the flight recorders from the EgyptAir jetliner that crashed in the Mediterranean and killed all 66 people aboard, while hundreds of Coptic Christian mourners filled a church in Cairo to pray for their relatives among the dead.

Mounting evidence pointed to a sudden and dramatic catastrophe that led to Thursday’s crash of Flight 804 from Paris to Cairo, although Egyptian President Abdel-Fattah el-Sissi said it “will take time” to establish what happened aboard the Airbus A320.

In his first public comments since the crash, el-Sissi cautioned against premature speculation.

“It is very, very important to us to establish the circumstances that led to the crash of that aircraft,” el-Sissi said in remarks broadcast live on Egyptian TV. “There is not one scenario that we can exclusively subscribe to. ... All scenarios are possible.”

A submarine belonging to the Oil Ministry was headed to the site about 180 miles north of the Egyptian port of Alexandria to join the search, el-Sissi said. The vessel can operate at a depth of 9,800 feet, he said.

After starting his comments with a minute of silence to remember the victims, he thanked the nations that have joined Egyptian ships and aircraft in the search.

Beside Egypt, ships and planes from Britain, Cyprus, France, Greece and the United States are taking part in the search for the debris from the aircraft, including its flight data and cockpit voice recorders. Some wreckage, including human remains, have been recovered already.

Egypt’s aviation industry has been under international scrutiny since Oct. 31, when a Russian Airbus A321 traveling to St. Petersburg from the Red Sea resort of Sharm el-Sheikh crashed in the Sinai Peninsula, killing all 224 people aboard. Russia said the crash was caused by a bomb planted on the plane, and the local branch of the Islamic State claimed responsibility, citing Moscow’s involvement in Syria.

Thursday’s crash will further damage Egypt’s tourism industry, already reeling from years of political turmoil. The nation of 90 million people has been in crisis after crisis since a popular 2011 uprising toppled autocrat Hosni Mubarak. Since then, it has seen a dramatic surge in attacks by Islamic militants, bouts of deadly unrest, a battered economy and the steady decline in the value of its currency.

El-Sissi spoke a day after the leak of flight data indicated a sensor detected smoke in a lavatory and a fault in two of the plane’s cockpit windows in the final moments of the flight. The data was published by The Aviation Herald.

Authorities say the plane lurched left, then right, spun all the way around and plummeted 38,000 feet into the sea, never issuing a distress call.