A Coptic Christian grieves during prayers for the departed, remembering the victims of Thursday's crash of EgyptAir flight 804, at Al-Boutrossiya Church, the main Coptic Cathedral complex, in Cairo, Egypt, May 22, 2016. 
Photo by Amr Nabil/AP

Egypt sends submarine to search for EgyptAir Flight MS804 black boxes

Updated

Egypt deployed a submarine on Sunday to join the search for EgyptAir Flight MS804’s black boxes amid the ongoing investigation into what downed the passenger plane.

President Abdel-Fattah al-Sisi announced the deployment of the submarine Sunday in his first public remarks about the crash, warning that an investigation into the incident will take time.

“Until now all scenarios are possible. So please, it is very important that we do not talk and say there is a specific scenario,” Sisi said, according to Reuters. “This could take a long time.”

Sisi’s remarks came as an audio recording of the EgyptAir pilot and air-traffic controllers emerged that suggested nothing was amiss 2 ½ hours before the jet disappeared from radar screens over the Mediterranean Sea.

RELATED: What Triggered Smoke Alerts on Doomed Flight?

“Hello, hello, EgyptAir 804, flight level 370, squawk number 7624,” the pilot of EgyptAir MS 804 is heard telling Zurich Airport in Switzerland in audio recorded by the website liveatc.net at around 11:51 p.m. Wednesday local time.

“EgyptAir 804 radar contact,” an air-traffic controller says, according to the audio. The pilot replies, “Thank you so much.”

The plane was traveling from Paris to Cairo when it disappeared with 66 people on board shortly after leaving Greek airspace at around 2:30 a.m. Thursday Cairo time (8:30 p.m. Wednesday ET).

Egypt’s military said it has found debris from the plane along with some human remains and passengers’ belongings.

Greek authorities have previously said the pilot appeared cheerful in communications over that country, thanking air traffic control in Greek as the plane prepared to leave Greek airspace.

Why the plane went down remains unclear.

Smoke was detected aboard EgyptAir Flight MS804 before it crashed but no conclusions are being drawn about the cause, France’s air accident investigation agency said Saturday.

The plane sent automated messages indicating smoke a few minutes before it disappeared from radar into the Mediterranean Sea, BEA spokesman Sebastien Barthe told NBC News.

“This usually means a fire,” he said.

This story first appeared on NBCNews.com.

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Egypt sends submarine to search for EgyptAir Flight MS804 black boxes

Updated