A murder on the streets of Belfast has triggered a massive political crisis in Northern Ireland.
There are fears the whole power-sharing agreement could collapse.
Newsbeat explains why this killing is causing so many problems. To understand what's going on now, you have to go back in time.
Ninety years ago Ireland was split in two after people living there went to war against their British rulers.
The south became a separate state, now called the Republic of Ireland.
But the break-up led to decades of unrest and violence in Northern Ireland, which remained part of the UK.
Broadly speaking, there are two main sides.
- Nationalists or republicans, who are mainly Catholic, believe the north should join a united, independent Ireland.
- Unionists or loyalists, who are often Protestant, think Northern Ireland should stay part of the United Kingdom.
The period known as "The Troubles" began in the late 1960s and lasted for nearly 30 years.
British troops were deployed to Northern Ireland, at first to protect Catholics, but soon became involved in bursts of fierce fighting with paramilitary groups.
More than 3,600 people from both sides were killed. A further 50,000 others were injured.
Republican militant groups (paramilitaries) like the IRA and INLA wanted Northern Ireland to join with the Republic of Ireland. There were also splinter groups called the Provisional and Real IRA.
Loyalist paramilitaries, such as the UDA/UFF and UVF, wanted the union to remain in place.
The search for peace
The Good Friday Agreement was signed in 1998 and was seen as a major step towards peace in Northern Ireland.
It laid plans for paramilitaries to hand over their weapons and for some prisoners to be released.
The deal also saw the creation of the Northern Ireland Assembly, a coalition that forces both sides to work together.
But because of the violence of the past, decisions are often made slowly because everyone has to be happy.
Over the years massive steps forward have been made. Critically for most people, the fear of violence and bombings has all but gone.
But tensions remain. Belfast is still divided into neighbouring Catholic and Protestant pockets.
The lines between them are sometimes marked with "peace walls". These high barriers, with fences on top, have gates that are locked at night to keep rival communities apart.
Why is there a new crisis?
The murder of a former IRA man has seen the tensions at the Northern Ireland Assembly bubble right to the surface.
The police service in Northern Ireland says current Provisional IRA members could be involved in the killing.
That's a big problem for the political party Sinn Fein. It's had links with the IRA but says the police claims are false and insists the Provisional IRA no longer exists.
That answer's not good enough for Northern Ireland's First Minister Peter Robinson. He leads the Democratic Unionist Party (DUP).
Unionists argue that if the IRA is still active, Sinn Fein should not be allowed to have ministers in government.
Peter Robinson refused to carry on working with Sinn Fein and asked the Assembly to be adjourned or suspended so talks can be held to discuss the murder.
But that idea was rejected.
Peter Robinson's now resigned and all but one of the DUP's ministers have stood down too.
That could mean a total collapse of power-sharing in Northern Ireland and lead, once again, to the country being ruled from London.
And that's what caused the violence in the first place.