A leaked audio recording has revealed that former Prime Minister Sheikh Hasina personally authorised the use of lethal force during last year’s anti-government protests, according to a BBC investigation.

In the verified audio, Hasina says she authorised her security forces to "use lethal weapons" against protesters and that "wherever they find (them), they will shoot".

The leaked audio of a conversation between Hasina and an unidentified senior government official is the most significant evidence so far of her direct role in ordering action against anti-government demonstrators.

A source familiar with the matter told the BBC that the phone call took place on July 18 from Hasina’s official residence Ganabhaban.

It remains unclear who leaked the audio which surfaced in early March this year.

Since the protests, numerous clips of Hasina’s alleged calls have circulated online, many of which remain unverified.

Bangladesh’s Criminal Investigation Department (CID) has matched the voice in the July 18 recording with Hasina’s voice.

The BBC independently verified the authenticity of the audio using forensic experts from Earshot, a non-profit that specialises in audio investigations relating to human rights and environmental issues.

The experts found no signs of editing or manipulation, and determined that the possibility of the audio being artificially generated was very low.

Earshot stated that the leaked recording was likely captured in a room where the phone call was played on speaker, citing the presence of distinctive telephonic frequencies and background noises.

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Earshot identified Electric Network Frequency (ENF) throughout the recording, a frequency that's often present in audio recordings due to interference between a recording device and mains-powered equipment, an indicator that the audio has not been manipulated.

Earshot also analysed Sheikh Hasina's speech – the rhythm, intonation and breath sounds -- and identified consistent noise floor levels, finding no evidence of synthetic artefacts in the audio.

British international human rights lawyer Toby Cadman told the BBC, “The recordings are extremely important in demonstrating her (Hasina’s) role—they are clear, properly verified and corroborated by other evidence.”

Cadman is serving as an adviser to Bangladesh’s International Crimes Tribunal (ICT) where cases are ongoing against Hasina and others.

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An Awami League spokesperson said, "We cannot confirm whether the tape recording referenced by the BBC is authentic."

The ICT has so far charged 203 people, 73 of whom remain in custody.