Northern Ireland’s first minister, Peter Robinson, stepped down on Thursday and pulled most of his Democratic Unionist Party (DUP) ministers out of the region’s devolved government in an escalation of its standoff with the republican Sinn Féin.

The only unionist cabinet minister who did not resign, Arlene Foster, is taking over as acting first minister.

The resignations come after Robinson failed to gain the backing of other parties to suspend the Northern Ireland assembly and call early elections. His sudden withdrawal from the provincial government casts doubt on a power-sharing agreement that has kept the peace in Britain’s Irish enclave for twenty years.

The crisis was triggered in August when a former Irish Republican Army gunman, Kevin McGuigan, was shot, apparently by members of the same underground paramilitary organization that fought against British rule in Northern Ireland for decades.

McGuigan’s death was seen as retaliation for the murder of another former IRA member, Gerard Davison, in May.

Sinn Féin, the political wing of the republican movement, was implicated in McGuigan’s murder when Bobby Storey, a former IRA member himself and now a Sinn Féin official, was arrested this week. Police said on Thursday he had been released.

The party denies Storey was involved and accuses the unionists of creating a “fake crisis.” But it has failed to persuade them that the IRA has truly “gone away,” as it claims.

The smaller and more centrist Ulster Unionist Party (UUP) already pulled out of the executive earlier this month, a decision Robinson criticized at the time as “party politics.”

He wrote in the Belfast Telegraph, “One would have more respect for the decision if there were even a degree of honesty about the motivation behind it.”

The two parties compete for unionist votes. The more hardline DUP – which was the only major party to oppose the 1998 Good Friday Agreement that ended the armed struggle in Northern Ireland and established its devolved assembly and executive — has overtaken the UUP in recent years as the dominant party on the right.