A ferry carrying about 1,400 people, most of them Egyptians, has sunk in the Red Sea.
The al-Salam Boccaccio '98 was sailing in poor weather
The al-Salam Boccaccio '98 went down about 80km (50 miles) off the Egyptian coast during an overnight journey from Duba in Saudi Arabia to Safaga in Egypt.
The BBC News website investigates the areas which investigators are likely to consider as possible causes of the disaster.
The al-Salam Boccaccio '98 was operated by Egyptian company El-Salam Maritime Transport and there are conflicting reports on whether it met all regional safety requirements.
A spokesman for Egyptian President Hosni Mubarak said the speed at which the ship sank and the fact there were not enough life rafts on board confirmed that there was a (safety) "problem".
The 35-year-old ship, reported to have previously operated in Italy, was of a design that may not have been allowed to operate everywhere.
The ship was a "roll on-roll off" (ro-ro) ferry, on which vehicles drive on at one end of the ferry, and then off at the other - in contrast to ships where cargo must be winched aboard.
To allow vehicles to embark and disembark, ro-ro ships are equipped with large external doors close to the waterline at the front and rear of the vessel, which create a risk of flooding if they are not properly sealed.
Even if a small amount of water enters through the doors, it starts slopping from side to side, making the ship rock.
As the water moves it gathers momentum, causing the rocking to become more pronounced. The shifting water quickly makes the ship unstable and can capsize it within a short space of time.
This phenomenon - known as the Free Surface Effect - played a key role in the sinking of the Herald of Free Enterprise ferry off Zeebrugge in 1987, after it set sail with its bow doors open.
After this, and then the loss of the Estonia ferry in the Baltic Sea in 1994, killing more than 850 people, the EU beefed up its regulations on ro-ro ferries, but less stringent regulations continue to apply in some other parts of the world.
ALTERATIONS TO SHIP
Sources familiar with the ship say that it was modified in the 1980s, with two more passenger decks placed on the top of the vessel.
As a result, says David Osler from the London shipping paper Lloyds List, the ferry had an "unusually high profile", sitting much taller in the water than it was originally designed to do.
It is also known that there were high winds on the night that the ferry sank. Investigators will want to know whether the increased height of the ship combined with the strong winds could have been a factor in the disaster.
The possibility of human error is likely to be looked into as a standard part of any inquiry. Investigators will want to establish whether the doors were properly sealed and whether all other routine tasks had been carried out.
They are also likely to check whether the ballast tanks were in an appropriate state.
Large ships are equipped with the tanks - compartments within the hull which hold water. Typically, one tank is near the centre of the ship, or multiple tanks are placed on either side.
The weight of the water helps the ship maintain a fixed centre of gravity, despite the motion of the waves, keeping it stable.
Having empty tanks, known as "slack tanks", or partially filled tanks with water sloshing around, alters the ship's centre of gravity as it rolls with the motion of the waves, greatly reducing its stability and making it prone to capsizing.
Along with the 1,400 passengers and crew the ferry was carrying about 220 vehicles. If one or more of these vehicles shifted suddenly during the voyage, possibly due to the bad weather the ship was sailing in, it could theoretically have pierced the side of the hull from the inside.
The ship was travelling between Duba in Saudi Arabia and Safaga in Egypt, which face each other across the Red Sea. This is one of the busiest shipping areas in the world, with traffic not only travelling east and west between Saudi Arabia and Egypt, but also north-south through the Suez Canal, which links the Red Sea with the Mediterranean.
However, no other vessel has been reported missing in the area, making this an unlikely cause.
TERRORISM OR PIRACY
Al-Qaeda attacks on a US Navy ship in the Jordanian port of Aqaba in August 2005 and on the USS Cole in Yemen in 2000 have prompted fears that the terror group could try to attack a passenger ship at some point.
And in November 2005 assailants on speedboats, armed with rocket propelled grenades, attacked a luxury cruise liner off the coast of Somalia, highlighting the dangers of piracy.
However, experts say there is no indication that either terrorism or piracy are linked to the sinking of the al-Salam Boccaccio '98.