CAIRO — A party of ultraconservative Islamists that emerged as an unexpected political kingmaker in Egypt after the military’s ouster of President Mohamed Morsi said on Monday that it was suspending its participation in efforts to form an interim government.

A spokesman for the Al Nour party said its decision was a reaction to a “massacre” hours earlier at an officers’ club here in which security officials said more than 50 people had been killed. The decision brought new complexities and unanswered questions to the effort to create a transitional political order.

The Al Nour party was the only Islamist party to support removing Mr. Morsi, despite his ties to the more moderate Islamists of the Muslim Brotherhood. And the sight of Al Nour’s bearded sheik, standing behind the general who announced the takeover on television, was the only signal to Egyptian voters that the move had not been an attack on Islam, as some of the ousted president’s supporters are saying.

The party played a starring role in the military’s choreographed presentation of its takeover as the chance to reunify a country on the brink of civil war between opponents and supporters of Mr. Morsi’s Muslim Brotherhood. But while Al Nour’s leaders say they intend to build bridges, some liberals say the party is pushing potentially divisive demands, from picking a new prime minister to keeping Islam prominent in any new constitution.