The US is due to unseal charges against a Libyan man suspected of making the bomb that blew up Pan Am Flight 103 over Lockerbie in 1988, US media say.
They say US prosecutors will soon press for the extradition of Abu Agila Mohammad to stand trial in the US.
He is currently being held in Libya, according to the Wall Street Journal. This has not been confirmed by the Libyan authorities.
The terrorist attack killed 270 people over the Scottish town.
Most of the victims on board the flight from London to New York were American citizens.
Libyan national Abdelbaset al-Megrahi was the only man convicted over the bombing in 2001.
Megrahi, who always proclaimed his innocence, unsuccessfully appealed against his conviction.
But he was subsequently allowed to return home after it emerged that he had terminal cancer. He died in 2012.
The town scarred by Pan Am flight 103
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- Timeline: Lockerbie bombing
What are US media reporting?
The US Department of Justice is expected to unseal a criminal complaint against Mr Masud in the coming days, the Wall Street Journal is reporting quoting senior department officials.
It says this will open a new chapter in one of the world’s longest and most sprawling terrorism investigations.
Meanwhile, the New York Times says Mr Masud’s exact whereabouts are unknown. But it adds that he was jailed in Libya at one point for unrelated crimes.
Mr Masud is alleged to have been a top bomb-maker for late Libyan dictator Muammar Gaddafi.
Neither the US justice department nor the Libyan authorities have publicly commented on the issue.
Lockerbie bombing – timeline
US and British investigators indicted Megrahi in 1991 but he was not handed over by the Libyans until April 1999.
May 2000 – A special trial under Scots law starts on neutral ground at Camp Zeist in the Netherlands.
31 January 2001 – Former Libyan intelligence officer Megrahi is found guilty of mass murder and jailed for life with a minimum term of 27 years.
March 2002 – Megrahi loses an appeal against his conviction.
September 2003 – The Scottish Criminal Cases Review Commission (SCCRC) is asked to investigate Megrahi’s conviction.
June 2007 – The SCCRC recommends that Megrahi is granted a second appeal against his conviction.
18 August 2009 – Megrahi’s move to drop his second appeal is accepted by judges at The High Court in Edinburgh.
20 August 2009 – Megrahi, who has terminal prostate cancer, is released from prison on compassionate grounds.
May 2012 – Megrahi dies at his home in Tripoli, aged 60.
July 2015 – Scottish judges rule that relatives of the Lockerbie bombing victims should not be allowed to pursue an appeal on Megrahi’s behalf. Courts had previously ruled that only next of kin could proceed with a posthumous application.
July 2017 – Megrahi’s family launched a new bid to appeal against his conviction.
March 2020 – The Scottish Criminal Case Review Commission said Megrahi’s conviction can be taken to a fresh appeal.