A pair of explosions at a Beirut, Lebanon, port on Tuesday has left more than 100 people dead, more than 4,000 injured and scores missing.
As the investigation into what happened and the search for the missing continues, here is what we know about the explosions.
What happened on Tuesday?
On Tuesday evening at 6:07 p.m. local time near Beirut's port, there were two explosions, the second much larger than the first.
The second blast produced a shock wave that shattered glass, knocked over cars and caused damage to buildings up to six miles away. It sent a billowing, orange/red cloud into the sky over the city as it devastated the shopping and nightlife districts near the port and so damaged a hospital that it had to be closed and its patients moved to other facilities.`
The port sits north of the city center, on the eastern part of the Saint George Bay on Beirut's northern Mediterranean coast.
What caused the explosions?
The cause of the explosions is not known yet, but it is believed that the first blast may have been in a fireworks warehouse. The second blast is likely to have happened in a warehouse next to that one. That warehouse contained 2,750 metric tons of ammonium nitrate, a chemical used as fertilizer and in mining.
According to Lebanon’s Prime Minister, Hassan Diab, the ammonium nitrate had been stored in the warehouse for the past six years.
"I will not rest until we find those responsible for what happened, hold them accountable, and impose maximum punishment," he said in a statement, saying it was "unacceptable" that so much ammonium nitrate had been stored "while endangering the safety of citizens."
What is ammonium nitrate?
Ammonium nitrate is one of the world’s most widely used fertilizers. The United States uses millions of tons of it a year.
It is also used in the making of explosives that are used in mining.
What causes it to explode?
Ammonium nitrate (AN) is not an explosive on its own, but when it has a catalyst, the blast that results can be devastatingly large.
AN does produce heat when it decomposes. If enough is stored in one location, the heat that is produced can cause the AN to catch fire. That fire causes the ammonium nitrate to produce oxygen. The oxygen then feeds the fire. The AN then begins to fuse together and create a seal. Gasses form behind the seal and begin to expand. Eventually, the gas builds up enough to break through the seal and cause an explosion.
The explosion is so strong, it causes the AN to vaporize and a large amount of oxygen gas is formed.
Does the color of the smoke give us a clue as to what happened?
Anthony May, a retired ATF explosives investigator, told CNN that the color of the cloud after the blast suggests there may have been other compounds involved.
Robert Baer, a former CIA operative with extensive experience in the Middle East, told CNN "that the orange ball (of fire)" indicated possible military-grade explosives.
Jimmie Oxley, a chemistry professor at the University of Rhode Island, told AFP that the explosion could have been a result of the AN being stored improperly.
"If you look at the video [of the Beirut explosion], you saw the black smoke, you saw the red smoke, that was an incomplete reaction," she said.
“I am assuming that there was a small explosion that instigated the reaction of the ammonium nitrate - whether that small explosion was an accident or something on purpose I haven’t heard yet.”
How big were the blasts?
While the first explosion was big, witnesses say the second explosion was so big it felt like an earthquake.
In fact, the explosion registered as a 3.3 magnitude earthquake on Richter scales and was felt across the region, even as far as Cyprus, more than 100 miles away.
Has ammonium nitrate played a part in explosions before?
Explosions attributed to AN have happened both on purpose and as an accident. In 1947, a ship carrying about 2,000 tons of AN exploded in the port of Texas City, Texas. The blast killed 581 people.
Timothy McVeigh devised a bomb using AN and ignited it to blow up the federal building in Oklahoma City in 1995, killing 168 people.