On 21 December 1988 a Pan AM flight from London to New York exploded over Lockerbie, a town in the south of Scotland.
All 259 passengers on board the plane were killed as well as 11 people on the ground.
It's the biggest terrorist attack ever to take place in Britain.
After the bombing
Abdelbaset al-Megrahi was a Libyan intelligence officer who was charged with planning the bombing, though he insisted he was innocent.
It took over 12 years for him to be found guilty of planning the attack and he was sentenced to life in prison in Scotland.
At that time the Libyan government accepted responsibility for the bombings and said that they believed the right man was in jail.
Al-Megrahi's early release
In 2009 al-Megrahi was released early from prison because he was ill with cancer and doctors thought he only had a few months left to live.
Relatives of the bombing victims criticised the decision to release him from prison, but other people still believe he's innocent.
British Prime Minister David Cameron said that he should never have been released.
He returned to Libya but lived for another three years. He died aged 60 on 20 May 2012.
So why is it complicated?
Al-Megrahi has always said that he wasn't responsible for the bombings, and even some of the victims' families don't think he was guilty.
No-one else has ever been charged and there are big questions about whether al-Megrahi would have been able to organise the attack on his own.
Some people want the investigation into who was responsible to be re-opened. They believe al-Megrahi was wrongly convicted and say there are still questions over how and why the bombing happened.