The surprise decision was confirmed on Monday by several judicial sources, one of whom said Investigator Judge Tarek Bitar had issued new charges against eight senior and mid-ranking officials but did not identify them.

The investigation into the explosion that killed 220 people and shattered Beirut had been derailed by legal challenges and resistance from factions including the heavily armed Iran-backed Hezbollah.

"It (the resumption of the inquiry) was a beautiful shock but we want to be sure that no one stands in his way," William Noun, who lost his brother in the explosion, told Reuters.

Mariana Fadoulian, whose her sister was killed in the blast, said: "This made us so happy."

The explosion on Aug. 4, 2020 was caused by hundreds of tonnes of ammonium nitrate that had been left unloaded at the port in 2013, but no senior official has been held to account.

The sources said Bitar had ordered the release of five people detained since shortly after the blast -- former port officials Michel Nahoul, Shafiq Merhi and Sami Hussein; a welder of Syrian origin, Ahmad Rajab; and a contractor, Salim Shebli.

Politicians Bitar sought to question, including Hezbollah allies, made dozens of legal challenges disputing his right to interrogate them and saying he had overstepped his powers.

The investigation was paralysed in early 2022 by the retirement of judges from a court that must rule on several such complaints against Bitar before he can continue.

The court has been awaiting the appointment of new judges to resume its work - a step authorities have not taken.

Bitar resumed his work on the basis of a legal interpretation challenging the reasons for its suspension, the judicial sources said.

ANALYST SEES NEW TENSIONS

Hezbollah has campaigned against Bitar as he sought to question its allies and accused Washington of meddling in the probe. Hezbollah leader Sayyed Hassan Nasrallah called for him to be replaced.

An anti-Bitar protest called by Hezbollah and its allies in Oct. 2021 escalated into deadly violence.

The resumption of the investigation "will certainly raise tensions in the country because the factions who were opposed to his moves will raise hell - they will object again", said Nabil Boumonsef, deputy editor-in-chief of Annahar newspaper.

Bitar met French judges visiting Beirut last week as part of an investigation into the explosion opened by the French prosecutor's office because French nationals were among the casualties.

A senior Lebanese judicial source said he had been unable to share documents with them because the investigation was frozen.

Senior politicians Bitar sought to interrogate included members of Parliament Speaker Nabih Berri's Amal Movement, an important Hezbollah ally, along with Hassan Diab - prime minister at the time of the blast - and top security official Major-General Abbas Ibrahim.

All of them, including former ministers Ali Hassan Khalil and Ghazi Zeaiter, have denied wrongdoing, and some say they have immunity from prosecution.

The Lebanese judiciary, where appointments depend largely on political backing, has long been prone to political influence.

In November, Lebanon's most senior judge said political meddling in judicial work had led to a chaotic situation that required a "revolution in approaches" to resolve.

(Reporting by Laila Bassam; Writing by Timour Azhari and Tom Perry; Editing by Jon Boyle, Philippa Fletcher and Timothy Heritage)

By Laila Bassam, Maya Gebeily and Timour Azhari