Photo

Martin McGuinness, center, with party members Bobby Storey, left, and Martina Anderson speaking during rally last year in Northern Ireland. Mr. Storey has been arrested in connection with the slaying of a onetime I.R.A. member.Credit Peter Morrison/Associated Press

DUBLIN — The fragile provincial government in Northern Ireland teetered on the brink on Wednesday evening after the arrest of a senior member of Sinn Fein, the Irish nationalist party that shares power with British unionists, in connection with a murder last month that the police have linked to Irish Republican Army operatives.

Peter Robinson, leader of the pro-British Democratic Unionist Party and first minister of the province, threatened to withdraw his party from the Northern Ireland Assembly, the provincial Parliament, within 24 hours unless its members voted to suspend business until the murder investigation is resolved or the British government did so unilaterally. The Democratic Unionists hold the biggest bloc of seats, and their withdrawal would lead to a government collapse.

“The D.U.P. has made it clear it will not be involved in business as usual,” Mr. Robinson said in a statement. Unionists accuse Sinn Fein, the I.R.A.’s former political wing and their partner in government, of breach of trust.

His ultimatum followed the arrest of Bobby Storey, a Sinn Fein leader and former I.R.A. commander, in the killing of Kevin McGuigan, 53, a former I.R.A. member and a father of nine who was shot outside his home in Belfast on Aug. 12. With the arrest of Mr. Storey and two other former I.R.A. members on Wednesday, 16 people have now been detained in connection with the killing, which the police say they believe was an act of revenge for the assassination of Gerard (Jock) Davison, reportedly a onetime Belfast commander of the I.R.A., in May.

Mr. Storey is by far the highest-profile Sinn Fein politician to be questioned in the case, and is considered by many to be the right-hand man of Gerry Adams, the party president.

After Mr. McGuigan’s death, George Hamilton, the Northern Ireland police chief, asserted that despite the I.R.A.’s declaration renouncing violence and its formal disarming in 2005, I.R.A. structures and operatives still existed.

Sinn Fein immediately countered the assertions, saying the I.R.A. no longer existed. “The war is over,” Mr. Adams said in a statement. “The I.R.A. is gone and not coming back.”

Although Chief Hamilton said that there was no evidence that the killing of Mr. McGuigan had been sanctioned by senior members, and that intelligence indicated that the I.R.A. had ceased its paramilitary activities, the claim that the organization was still operating in any capacity incensed pro-British unionists. Mr. Robinson said Sinn Fein’s repeated denials that former I.R.A. members were involved in the murders had further undermined unionist confidence in the political process.

In a statement, Charlie Flanagan, the minister for foreign affairs and trade, urged a speedy resolution to the impasse.

“I urge Northern Ireland’s political leaders to pull back from the brink,” he said. “A collapse of the power-sharing institutions will serve no useful purpose.”

Mr. Storey, 59, spent more than 20 years in prison for a variety of arms offenses during the Northern Ireland conflict and is credited with helping mastermind a mass escape of I.R.A. prisoners from the top-security Maze Prison in 1983, an episode that embarrassed the British government.

Since his release in 1998, he has been vocal in his support of the continuing peace process, and he is Sinn Fein’s northern chairman, regarded as a highly influential role in the party.

Of the 16 people arrested in connection with Mr. McGuigan’s murder so far, 12 have been released, three are still being held, and one has been charged with weapons offenses.

In a statement on Wednesday, Northern Ireland’s deputy first minister, Martin McGuinness, said he was “surprised to learn about the arrest,” describing Mr. Storey as “a valued member of Sinn Fein’s core leadership.”