Garrett Discovery, a US-based firm that conducted the forensic analysis of an audio clip, allegedly of a conversation between former chief justice of Pakistan Mian Saqib Nisar and an unidentified person, has said it has received a threatening phone call for its work.

"Today we received a call saying our lives are in danger and the same person said he is going to file in court against us for our work authenticating a file for Fact Focus. 1000+ calls and chat requests on our site. Threatening our team to obtain a different result is unethical," it said via Twitter.

However, they did not specify where the call was from.

Although the account that tweeted about the threat is not verified, the firm's website directs visitors to that very account on Twitter.

Garrett Discovery first came into the limelight when the Fact Focus website published a news story along with an audio clip it attributed to former chief justice Nisar.

In the audio clip, which has not been independently verified by Dawn, a voice, allegedly of the former chief justice, can be heard saying: "Let me be a little blunt about it. Unfortunately, here it is the institutions that dictate judgements. In this case, we will have to punish Mian sahab (Nawaz Sharif). [I] have been told 'we have to bring Khan sahab (Imran Khan) [into power].'"

He allegedly goes on to add, "Punishment will have to be given."

For his part, Nisar termed the audio clip "fabricated", adding: "I have never spoken to the person in the audio call."

According to Fact Focus, the audio clip was examined by Garrett Discovery.

"Garret Discovery has a team of leading experts who have a long experience of analysing and presenting evidence and testifying before courts in the US. The firm's analysis report certifies the integrity of the audio file and states that 'this audio has not been edited in any way,'" according to the Fact Focus report.

Two days after the audio clip was shared, Garrett Discovery said that it had been contacted by over 500 journalists and individuals, adding that "Mr Nelson did the work so please quit saying Mr Bedford did the work."

It was unclear what exactly the firm was referring to.

According to the firm's website, Anthony Nelson — an audio and video forensic expert — holds a Master's degree in media forensics from the University of Colorado.

Nelson previously worked for the University of Colorado Denver's MediFor programme which was aimed at testing Defence Advanced Research Agencies (Darpa) media authentication software, it stated.

'Will share details if permitted'

A representative of the company earlier told Dawn, "We did perform work for Fact Focus. As part of our standard agreement, we have a confidentiality clause that prohibits us from speaking about the scope of work or work we performed.

"We have no problem releasing details as long as our client signs an agreement allowing us to do so. Please contact Fact Focus with any inquiries as to the work we performed and discuss with them any release of information. We will not comment anymore on this issue until that time," it added.

When Dawn asked Fact ­Focus if Garret Discovery also determined this was "a genuine audiotape of a conversation between two persons", they said they never made any claim about the authenticity of the conversation. "The forensic was about the tape, not who the two persons were. Although the first voice is that of Nisar, we don't know the second person."

Read: US forensic firm mentioned in report about leaked audio clip says will share details if permitted

The firm later tweeted that it had been contacted by Samaa TV — which published a report saying that the audio clip "appears to be pieced together from two separate speeches" of the former chief justice.

"Saama.tv called and we are not going to answer questions about our client, the work or make an opinion over the phone," the firm insisted.