For at least six years, hundreds of tonnes of ammonium nitrate, which Lebanese authorities say caused Tuesday's massive blast, were negligently stored in a Beirut port warehouse, waiting for disaster to strike.

The odourless crystalline substance commonly used as a fertiliser has caused numerous industrial explosions over the decades -- including the massive one in Beirut that killed at least 113 people, wounded thousands and left 300,000 homeless.

A security source said the explosive power of the stored ammonium nitrate was equivalent to at least 1,200 tonnes of TNT -- explaining how the earthquake-strength blast destroyed or damaged so much of the city.

Lebanese port authorities and customs officials knew the chemical was being stored in the port, and one of the country's top security agencies had called for it to be relocated after launching a probe last year, several security officials told AFP.

These satellite images by Airbus DS shows a view of the port of Beirut on January 25 and on August 5, a day after the blast sowed devastation across the city
These satellite images by Airbus DS shows a view of the port of Beirut on January 25 and on August 5, a day after the blast sowed devastation across the city CNES / -

But authorities did not heed the warning. Only on the day after the massive blast left much of the capital in ruins did the government say it was seeking house arrest for all officials involved in storing the highly-explosive substance.

With Tuesday's blast damage extending across half the Lebanese capital, the burning questions on everyone's mind are: how did so much ammonium nitrate get to Beirut in the first place and why was it stored at the port for so long?

In 2013, around 2,750 tonnes of ammonium nitrate came into Lebanon on board the Rhosus ship, sailing from Georgia and bound for Mozambique, a security official told AFP, asking not to be named because he is not authorised to speak on the issue.

The damaged grain silos of Beirut's harbour and its surroundings one day after a powerful twin explosion tore through Lebanon's capital
The damaged grain silos of Beirut's harbour and its surroundings one day after a powerful twin explosion tore through Lebanon's capital AFP / STR

Marine Traffic, a ship tracking platform, said the Moldova-flagged vessel first arrived in Beirut's port, the country's busiest, on November 20, 2013 and never left.

According to Lebanese law firm Baroudi & Associates, which represents the vessel's crew, the Rhosus ship had faced "technical problems".

"Upon inspection of the vessel by port state control, the vessel was forbidden from sailing," the firm said in a statement.

Several security officials told AFP that it temporarily docked at the port but was later seized by authorities following a lawsuit filed by a Lebanese company against the shipowner.

An aerial view shows the massive damage done to Beirut port's grain silos and the area around it
An aerial view shows the massive damage done to Beirut port's grain silos and the area around it AFP / -

Port authorities unloaded the ammonium nitrate and stored it in a rundown port warehouse with cracks in its walls, and the ship sank some time later because of damage, the officials said.

The warehouse started to exude a strange odour, which led security forces to launch a 2019 investigation that concluded that the "dangerous" chemicals needed to be removed from the premises.

The agency also noted the walls of the warehouse were unsound, urging the port authorities to repair them.

It was not until this week that workers were dispatched and began repair works, in what might have possibly triggered the blast.

Shortly after Tuesday's blast, the director of customs at the port, Badri Daher, published a letter he said he had sent in December 2017 to a Lebanese prosecutor, claiming it was one of many he had sent to the judiciary over the stored chemicals.

In the 2017 letter, he allegedly requested the dangerous chemicals be exported or sold to a local Lebanese company after the army had said it had no use for them, but neither suggestion materialised.

A judicial source said prosecutors were only involved in ruling whether or not the ammonium nitrate-carrying vessel should be released and were not involved in issues pertaining to the substance's storage.

Riad Kobaissi, an investigative reporter who specialises in port corruption, charged that Daher was only trying to deflect blame by publishing the letter.

He said what has happened shows "the extent of corruption in Lebanese port customs, which is among the main bodies that bear responsibility" for the blast.

Ammong many, the disaster has only fuelled anger at a government already widely seen as inept, corrupt and beholden to sectarian interests.

On Twitter, users blamed authorities, using the hashtag "hang them from the gallows".

One user posted a photo showing several prominent Lebanese politicians with the caption: "You have to pay for burning the hearts of mothers and the future of the youth and terrorising children."