The blast was one of the world's largest non-nuclear explosions, destroying swathes of the Lebanese capital, killing more than 220 people and injuring more than 6,500.

The Lebanese judicial official told AFP on condition of anonymity that "Bitar headed to Sofia on Wednesday" and is expected to question Grechushkin the following day.

The Lebanese embassy in Sofia is arranging for a translator and a clerk to record the minutes of the questioning, which Bulgarian judicial officials are to attend, the official said.

According to Bulgarian prosecutors, Grechushkin is accused by Lebanese judicial authorities of "introducing explosives into Lebanon -- a terrorist act that resulted in the death of a large number of people".

The Lebanese judicial official told AFP that authorities are relying on Grechushkin's testimony and the information he has about the ammonium nitrate shipment "and the party that ordered and financed it", and to determine if Beirut was the ship's destination.

Bitar resumed his investigation this year as Lebanon's balance of power shifted following a war between Israel and Hezbollah that weakened the militant group, which had spearheaded a campaign against him.

A travel ban imposed on Bitar as part of a judicial battle related to the case was recently lifted.

President Joseph Aoun and Prime Minister Nawaf Salam, who both took office this year, have vowed to uphold the independence of the judiciary in a country plagued by official impunity.

Officials named in the port explosion investigation had filed a flurry of lawsuits seeking to hamper its progress.