• A document seen by Reuters reveals that both prime minister and president knew the security risk the 2 750 tonnes ammonuim nirate would pose.
• "I warned them that this could destroy Beirut if it exploded," an official said.
• The rebuilding of Beirut is expected to cost up to $15 billion.

Lebanese security officials warned the prime minister and president last month that 2 750 tonnes of ammonium nitrate stored in Beirut's port posed a security risk and could destroy the capital if it exploded, according to documents seen by Reuters and senior security sources.

To recap, just over two weeks later, the industrial chemicals went up in a massive blast that obliterated most of the port and swathes of the capital, killed at least 163 people, injured 6 000 and destroyed 6 000 buildings, according to municipal authorities.

A report by the General Directorate of State Security on events leading up to the explosion included a reference to a private letter sent to President Michel Aoun and Prime Minister Hassan Diab on 20 July.

While the content of the letter was not in the report seen by Reuters, a senior security official said it summed up the findings of a judicial investigation launched in January which concluded the chemicals needed to be secured immediately.

The state security report, which confirmed the correspondence to the president and the prime minister, has not previously been reported.

"There was a danger that this material, if stolen, could be used in a terrorist attack," the official told Reuters.

"At the end of the investigation, Prosecutor General (Ghassan) Oweidat prepared a final report which was sent to the authorities," he said, referring to the letter sent to the prime minister and president by the General Directorate of State Security, which oversees port security.

"I warned them that this could destroy Beirut if it exploded," said the official, who was involved in writing the letter and declined to be named.

Reuters could not independently confirm his description of the letter.

The presidency did not respond to requests for comment about the 20 July letter.

A representative for Diab, whose government resigned on Monday following the blast, said the PM received the letter on July 20 and it was sent to the Supreme Defence Council for advice within 48 hours. 

"The current cabinet received the file 14 days prior to the explosion and acted on it in a matter of days. Previous administrations had over six years and did nothing."

The prosecutor general did not respond to requests for comment.

The rebuilding of Beirut is expected to cost up to $15 billion, in a country already effectively bankrupt with total banking system losses exceeding $100 billion.

Many questions remain over why the shipment of ammonium nitrate docked in Beirut in late 2013. Even more baffling is why such a huge stash of dangerous material, used in bombs and fertilisers, was allowed to remain there for so long.

The letter sent to Lebanon's president and prime minister followed a string of memos and letters sent to the country's courts over the previous six years by port, customs and security officials, repeatedly urging judges to order the removal of the ammonium nitrate from its position so close to the city centre.

The General Directorate of State Security's report seen by Reuters said many requests had been submitted, without giving an exact number. It said the port's manifest department sent several written requests to the customs directorate up until 2016 asking them to call on a judge to order the material be re-exported immediately.

"But until now, no decision has been issued over this matter. After consulting one of our chemical specialists, the expert confirmed that this material is dangerous and is used to produce explosives," the General Directorate of State Security report said.

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