The US has announced charges against Libyan man Abu Agila Mohammad Masud who is suspected of making the bomb that blew up Pan Am Flight 103 over Lockerbie in 1988 which killed 270 people.
Prosecutors will now seek his extradition to stand trial in the US.
Fellow Libyan Abdelbaset al-Megrahi was the only other man convicted over the bombing and was sentenced to 27-years imprisonment in 2001.
He maintained his innocence and was released from prison in Scotland and allowed to return to Libya on compassionate grounds in 2009 after it emerged that he had terminal cancer. He died three years later.
A Scottish court is now considering a posthumous appeal against his conviction by his family.
On the 32nd anniversary of the bombing, US Attorney General William Barr said: “I am pleased to announce that the United States has filed criminal charges against the third conspirator Abu Agila Mohammad Masud for his role in the bombing
“Let there be no mistake, no amount of time or distance will stop the United States and our Scottish partners in pursuing justice in this case.”
The breakthrough came when authorities learned that Masud was being held in Libya and Mr Barr said Libyan authorities provided a copy of their interview with him to US officials
According to Mr Barr, Masud allegedly built the bomb and worked with two other co-conspirators.
He added: “At long last this man responsible for killing Americans and many others will be subject to justice for his crimes.
Mr Barr said that he is “optimistic” that the Libyan government will hand Masud over.
Police Scotland Chief Constable Iain Livingstone said: “This announcement by the US Department of Justice is a significant development for the families of the victims, and my thoughts remain with them, particularly today, the 32nd anniversary of the bombing.
“Since 1988, policing in Scotland has been committed to carrying out the largest terrorist investigation ever undertaken in this country. Police Scotland will continue to work closely on this investigation, under the direction of the Crown Office, with our American law enforcement colleagues and other international partners.
“As judicial proceedings continue in Scotland, it would be inappropriate to comment further.”