By Timour Azhari, Al Jazeera

Beirut, Lebanon – Lebanon’s lead investigator in the August 4 Beirut explosion that killed more than 200 people has been informed that French experts have so far reached no conclusion on the cause of the blast, a senior Lebanese judicial source said.

Judicial investigator Fadi Sawan was provided with the preliminary information during a call on Tuesday with a French judge investigating the blast, the source told Al Jazeera.

The French judiciary has opened its own investigation into the explosion as French citizens were among those killed and injured.

In the wake of the August 4 explosion that officials have said was fuelled by some 2,750 tonnes of ammonium nitrate left at the port for nearly seven years, French, American and British experts were deployed to help Lebanese investigators.

The United States, via the FBI, has already provided Sawan with its findings, which reached no firm conclusion as to what caused the blast, according to a report by the Reuters news agency.

A French expert report is expected to arrive within the next week or so.

The source said the French judge told Sawan he had not yet officially been handed a report by French technical and security experts, but had reviewed some of the information it is set to contain.

“He stated clearly that the experts have not determined conclusively whether the explosion was the result of an intentional security operation or whether it was the result of negligence in storing the ammonium nitrate and shortcomings that led to the devastating explosion,” the source said.

Caretaker Justice Minister Marie-Claude Najem told Al Jazeera last week that Lebanese investigators were placing significant weight on the French expert report because it was expected to go into greater depth than the FBI’s, and Lebanon had a limited technical capability to reach its own conclusions on the cause of the blast.

The devastating explosion injured more than 6,500 people and caused damage estimated at billions of dollars to the capital city, initially leaving hundreds of thousands homeless.

Top political and security officials knew of the presence of the explosive substance for years before the blast, but none is officially considered a suspect, nor are they set to be indicted, judicial sources told Al Jazeera last week.

Instead, low and mid-level administrative and security officials have been charged with “willful negligence”, a crime with a maximum sentence of about five years in prison, sources said.

Meanwhile, Sawan is continuing his investigation and has begun hearing the testimony of some 50 victims or people otherwise affected by the explosion who have filed suit in a personal capacity.

He has also begun to receive hundreds of other suits filed by the Beirut Bar Association on behalf of victims late last month.