AL-QARDAHA, Syria—On the fourth day of a rebel assault on President Bashar al-Assad’s seat of power in Damascus, an explosion tore through offices of the National Security Bureau, killing the president’s brother-in-law and three other senior officials.

Rebel groups claimed credit for the audacious plot, and Syrian opposition groups declared it was the beginning of the end for the regime. In Washington, the Obama administration ordered a task force from the Pentagon, State and Treasury to draw up plans for a post-Assad Syria, said Robert Ford, the U.S. ambassador to Syria at the time.

The July 2012 bombing indeed marked a turning point in Syria’s conflict. But rather than the downfall of Mr. Assad, it ushered in a new, more deadly phase of Syria’s civil war that allowed him to cling to power. Any regime voices still open to accommodating the opposition went silent, and, within a year, pro-Assad forces deployed chemical weapons against rebels and civilians.

Now, new revelations point to a startling theory about the bombing that killed Assef Shawkat, an army general who was Mr. Assad’s brother-in-law and the deputy defense minister: It was an inside job.

Two dozen people, including past and current regime officials, opposition leaders, activists and rebels, and politicians in neighboring countries with ties to Mr. Assad told The Wall Street Journal the bombing grew out of a split between the Assad family and its hard-line allies on one side, and officials seeking negotiations with opposition groups on the other.