A Libyan man has been charged with being a "third conspirator" in connection with the Lockerbie bombing, US Attorney General William Barr announced.
Speaking at a press conference, on the 32nd anniversary of the atrocity, Mr Barr said Libyan national Abu Agila Mohammad Masud Kheir Al-Marimi has been charged for his role in the bombing of Pan Am flight 103, which killed all 270 people on board and 11 people on the ground.
Mr Barr said a “third conspirator” was identified along with two Libyan intelligence agents during the investigation in 1991 but at the time investigators could not “identify or locate” this person.
Mr Barr announced an earlier set of charges against two Libyan intelligence officials in his capacity as acting attorney general nearly 30 years ago, vowing that the investigation would continue.
Though Barr had not appeared at a press conference in months, he led this one two days before his departure as something of a career bookend.
"I am pleased to announce that the United States has filed criminal charges against the third conspirator," Mr Barr said. "For his role in the bombing of Pan Am flight 103.
"Let there be no mistake, no amount of time or distance will stop the United States and our Scottish partners from pursuing justice in this case."
The majority of people on board the flight, 190 in total, were American, including a group of college students returning home for Christmas. Citizens from 21 different countries perished in the attack.
Mr Barr added: "There is no question that the Pan Am attack was aimed at the United States and this heinous assault lives in infamy in the collective memory of the American people."
The 1988 terrorist atrocity was Britain's largest terrorist attack and the second worst in US history, Mr Barr said.
He added: "To the families of those who died in the sky over Lockerbie all those years ago, I know that the small step we take today cannot compensate for the sorrow you feel to this day.
"But I hope that you will find some measure of solace in knowing that we in the United States government on behalf of the American people and in partnership with our counterparts in Scotland have never relented and will never relent in the pursuit of justice for you and for your loved ones."
The father of one of the victims criticised the decision to make the announcement on the anniversary of the attack as "disrespectful".
Rev John Mosey, the father of 19-year-old victim Helga Mosey, said he considered the “timing and particularly the choice of this specific day, which is special to many of us, to be bizarre, disrespectful, insensitive and extremely ill considered”.
Former Libyan intelligence officer Abdelbaset al-Megrahi, who was found guilty in 2001 of mass murder and jailed for life with a minimum term of 27 years, was the only person convicted of the attack.
He was released from prison in 2009 on compassionate grounds and died in Libya in 2012.
The bombing of Pan AM Flight 103 spurred global investigations and produced sanctions against Libya, which ultimately surrendered intelligence officials wanted in the attacks for prosecution in Europe.
Scotland’s Lord Advocate James Wolffe QC said: “For 32 years the families of the 270 people murdered in this atrocity have shown extraordinary and enduring dignity in the face of the loss they suffered on the terrible night of December 21 1988. Today, our thoughts are with them once again.
“Scottish prosecutors and police have had a long-established and strong working relationship with US law enforcement agencies throughout this investigation.
“This relationship will continue to be important as the investigation progresses with the shared goal of bringing all those who committed this atrocity to justice.
“Scottish prosecutors will continue to work with US colleagues but we will not comment in detail on today’s announcement given that the Scottish criminal investigation is ongoing and there is an appeal before the court in relation to this crime.”