Ominous graffiti reading "we will bring this plane down" was written on the underside of EgyptAir flight MS804 which crashed in the Mediterranean Sea on Thursday, 19 May, anonymous sources revealed. The graffiti was scrawled two years ago, during a time a great political unrest in the country, and was attributed to activists upset at the government, rather than militant threats.

Other writing on the plane included insults to Egyptian president, Abdel Fattah al-Sisi. Written in Arabic, some of the writing played on the similarity between Sisi's name and the plane's model – SU-GCC. The words 'traitor' and 'murderer' were also written.

Officials, who spoke to the New York Times on condition of anonymity and were interviewed separately, said the graffiti was linked to the domestic political situation at the time. Similar examples of writing were seen around Cairo after Sisi ousted the democratically elected president, Mohammed Morsi, in 2013.

Data alerts from the final minutes of flight MS804 revealed smoke being detected in a number of places and problems with a cockpit window before systems shut down. The plane reportedly took two sharp turns before crashing into the sea.

Egypt today sent out a submarine to aid in the search for the wreckage. Though debris including baggage and body parts has been found, the main body of the plane is still missing. The plane's wreckage will contain two flight recorders, known as "black box" recorders, which register data from the flight and record cockpit transmissions.

It is still not known what caused the plane to suddenly hurtle into the sea but the black box may hold key information to unravelling the mystery.